diff --git a/Cargo.lock b/Cargo.lock
index 7fb22c4..5259cac 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,15 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "android_system_properties"
+version = "0.1.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ae221649c9976a6f6c56ae1facf410f3ddb33cc661c4b7b61020a912d4237fbc"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "anstream"
 version = "1.0.0"
@@ -58,6 +67,12 @@ version = "1.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1505bd5d3d116872e7271a6d4e16d81d0c8570876c8de68093a09ac269d8aac0"
 
+[[package]]
+name = "autocfg"
+version = "1.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f2032f911046de80f0a198e0901378627c33f59ea0ac00e363d481118bd70a53"
+
 [[package]]
 name = "axum"
 version = "0.8.9"
@@ -116,18 +131,47 @@ version = "2.13.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b588b76d00fde79687d7646a9b5bdf3cc0f655e0bbd080335a95d7e96f3587da"
 
+[[package]]
+name = "bumpalo"
+version = "3.20.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72f5acc6cb2ba439de613abc23857ec3d78374d8ed5ac84e9d11336e87da8649"
+
 [[package]]
 name = "bytes"
 version = "1.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc652a48c352aef3ea3aed32080501cf3ef6ed5da78602a020c991775b0aff04"
 
+[[package]]
+name = "cc"
+version = "1.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "005ec2760ca554fae18df7a11195552ec576cd665632a881bc011d5bb2fd4d80"
+dependencies = [
+ "find-msvc-tools",
+ "shlex",
+]
+
 [[package]]
 name = "cfg-if"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9330f8b2ff13f34540b44e946ef35111825727b38d33286ef986142615121801"
 
+[[package]]
+name = "chrono"
+version = "0.4.45"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1aa79e62e7697b8e29b513a68abacf485adcd1fe8284a4316c5ae868e6633327"
+dependencies = [
+ "iana-time-zone",
+ "js-sys",
+ "num-traits",
+ "wasm-bindgen",
+ "windows-link",
+]
+
 [[package]]
 name = "clap"
 version = "4.6.6"
@@ -159,7 +203,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn",
+ "syn 3.0.5",
 ]
 
 [[package]]
@@ -174,6 +218,12 @@ version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
+[[package]]
+name = "core-foundation-sys"
+version = "0.8.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "773648b94d0e5d620f64f280777445740e61fe701025087ec8b57f45c791888b"
+
 [[package]]
 name = "errno"
 version = "0.3.14"
@@ -184,6 +234,12 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "find-msvc-tools"
+version = "0.1.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3e0f1c7c3a72c66fd80abe965175f7523475c0489a87d3ff9d6e8c87d87a9d2d"
+
 [[package]]
 name = "form_urlencoded"
 version = "1.2.2"
@@ -289,6 +345,12 @@ version = "1.0.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "df3b46402a9d5adb4c86a0cf463f42e19994e3ee891101b1841f30a545cb49a9"
 
+[[package]]
+name = "humantime"
+version = "2.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "15cdd26707701c53297e2fa6afb323d55fbc1d0810c3aec078ae3ef0424c3c15"
+
 [[package]]
 name = "hyper"
 version = "1.11.1"
@@ -324,6 +386,30 @@ dependencies = [
  "tower-service",
 ]
 
+[[package]]
+name = "iana-time-zone"
+version = "0.1.65"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e31bc9ad994ba00e440a8aa5c9ef0ec67d5cb5e5cb0cc7f8b744a35b389cc470"
+dependencies = [
+ "android_system_properties",
+ "core-foundation-sys",
+ "iana-time-zone-haiku",
+ "js-sys",
+ "log",
+ "wasm-bindgen",
+ "windows-core",
+]
+
+[[package]]
+name = "iana-time-zone-haiku"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f31827a206f56af32e590ba56d5d2d085f558508192593743f16b2306495269f"
+dependencies = [
+ "cc",
+]
+
 [[package]]
 name = "is_terminal_polyfill"
 version = "1.70.2"
@@ -336,6 +422,17 @@ version = "1.0.18"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
 
+[[package]]
+name = "js-sys"
+version = "0.3.105"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ce57d20d1ea864ce2ac172ab472d409214f4fd359f0b2a2775abdf522e2af99e"
+dependencies = [
+ "cfg-if",
+ "futures-util",
+ "wasm-bindgen",
+]
+
 [[package]]
 name = "libc"
 version = "0.2.189"
@@ -396,6 +493,15 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "num-traits"
+version = "0.2.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "071dfc062690e90b734c0b2273ce72ad0ffa95f0c74596bc250dcfd960262841"
+dependencies = [
+ "autocfg",
+]
+
 [[package]]
 name = "once_cell"
 version = "1.21.4"
@@ -413,7 +519,9 @@ name = "ottyl"
 version = "0.1.0"
 dependencies = [
  "axum",
+ "chrono",
  "clap",
+ "humantime",
  "serde",
  "serde_json",
  "tokio",
@@ -482,6 +590,12 @@ dependencies = [
  "bitflags",
 ]
 
+[[package]]
+name = "rustversion"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf54715a573b99ac80df0bc206da022bcd442c974952c7b9720069370852e21f"
+
 [[package]]
 name = "ryu"
 version = "1.0.23"
@@ -521,7 +635,7 @@ checksum = "e7a5d71263a5a7d47b41f6b3f06ba276f10cc18b0931f1799f710578e2309348"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 3.0.5",
 ]
 
 [[package]]
@@ -560,6 +674,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "shlex"
+version = "2.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8fadd59c855ef2080decdef8ff161eb6661b86933c9d82e5ba29dc602a55aba"
+
 [[package]]
 name = "signal-hook-registry"
 version = "1.4.8"
@@ -598,6 +718,17 @@ version = "0.11.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"
 
+[[package]]
+name = "syn"
+version = "2.0.119"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "872831b642d1a07999a962a351ed35b955ea2cfc8f3862091e2a240a84f17297"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
 [[package]]
 name = "syn"
 version = "3.0.5"
@@ -640,7 +771,7 @@ checksum = "78773a2a397f451582ce068015985c33193cf6dea8b74d2a639fe457b2f07b0e"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 3.0.5",
 ]
 
 [[package]]
@@ -753,12 +884,110 @@ version = "0.11.1+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ccf3ec651a847eb01de73ccad15eb7d99f80485de043efb2f370cd654f4ea44b"
 
+[[package]]
+name = "wasm-bindgen"
+version = "0.2.128"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "aecb87a33d3b0c5e3b7aa46336eaf486cffafbd281b195e4c8b80d50df2351bf"
+dependencies = [
+ "cfg-if",
+ "once_cell",
+ "rustversion",
+ "wasm-bindgen-macro",
+ "wasm-bindgen-shared",
+]
+
+[[package]]
+name = "wasm-bindgen-macro"
+version = "0.2.128"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a690d511e3c1a8b3a55e33511e3c2c00c78415cd23650f32b808627f5696b9ed"
+dependencies = [
+ "quote",
+ "wasm-bindgen-macro-support",
+]
+
+[[package]]
+name = "wasm-bindgen-macro-support"
+version = "0.2.128"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "411e4887f0071ef2d2164a9d5fdf2d20efbef78fccd3a78b0c10a1dc5295e48a"
+dependencies = [
+ "bumpalo",
+ "proc-macro2",
+ "quote",
+ "syn 3.0.5",
+ "wasm-bindgen-shared",
+]
+
+[[package]]
+name = "wasm-bindgen-shared"
+version = "0.2.128"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "81941cd78d0c92026c33e5e01312845a4cb1e9af3407f9134b100dd03144103e"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "windows-core"
+version = "0.62.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b8e83a14d34d0623b51dce9581199302a221863196a1dde71a7663a4c2be9deb"
+dependencies = [
+ "windows-implement",
+ "windows-interface",
+ "windows-link",
+ "windows-result",
+ "windows-strings",
+]
+
+[[package]]
+name = "windows-implement"
+version = "0.60.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "053e2e040ab57b9dc951b72c264860db7eb3b0200ba345b4e4c3b14f67855ddf"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
+[[package]]
+name = "windows-interface"
+version = "0.59.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f316c4a2570ba26bbec722032c4099d8c8bc095efccdc15688708623367e358"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
+
 [[package]]
 name = "windows-link"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
 
+[[package]]
+name = "windows-result"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7781fa89eaf60850ac3d2da7af8e5242a5ea78d1a11c49bf2910bb5a73853eb5"
+dependencies = [
+ "windows-link",
+]
+
+[[package]]
+name = "windows-strings"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7837d08f69c77cf6b07689544538e017c1bfcf57e34b4c0ff58e6c2cd3b37091"
+dependencies = [
+ "windows-link",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.61.2"
diff --git a/Cargo.toml b/Cargo.toml
index 24df485..0ffc609 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,3 +10,5 @@ tower-http = { version = "0.6", features = ["fs", "cors"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 clap = { version = "4", features = ["derive"] }
+chrono = "0.4"
+humantime = "2"
diff --git a/src/filter_datetime.rs b/src/filter_datetime.rs
new file mode 100644
index 0000000..aa5cbae
--- /dev/null
+++ b/src/filter_datetime.rs
@@ -0,0 +1,324 @@
+//! Datetime parsing for the filter query language's timestamp comparisons.
+//!
+//! This module is intentionally standalone: it has no dependency on the engine/index
+//! types or the filter grammar module, so it can be developed, tested, and reused
+//! independently of the rest of the filter machinery.
+//!
+//! Three input forms are supported, all normalized to a single [`Timestamp`]:
+//!
+//! 1. Relative humantime, e.g. `5h`, `-3m` — resolved against a supplied "now" at
+//!    parse/eval time (parsed with the `humantime` crate).
+//! 2. Absolute time-of-day, e.g. `12:00` — resolves to that time on the current day.
+//!    **This module treats "current day" as the UTC calendar day of `now`, and resolves
+//!    the time-of-day in UTC.** (We use `chrono` for date/time parsing since no other
+//!    datetime crate was already a dependency.)
+//! 3. Full date/datetime literals, e.g. `2024-01-15` or `2024-01-15T12:00:00Z`
+//!    (RFC3339-ish). A bare date is interpreted as midnight UTC on that date.
+
+// Not yet wired into the filter grammar/engine (a separate integration ticket); allow
+// dead code so this standalone module builds warning-free until that lands.
+#![allow(dead_code)]
+
+use chrono::{DateTime, NaiveDate, NaiveTime, TimeZone, Utc};
+
+use crate::timestamp::Timestamp;
+
+/// Error returned when a datetime filter literal can't be resolved to a [`Timestamp`].
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum DateTimeParseError {
+    /// The input matched the shape of a supported format (relative duration,
+    /// time-of-day, or date/datetime) but its value was invalid or ambiguous,
+    /// e.g. an out-of-range time like `25:99`.
+    Invalid { input: String, reason: String },
+    /// The input didn't match any supported format at all.
+    Unparseable(String),
+}
+
+impl std::fmt::Display for DateTimeParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            DateTimeParseError::Invalid { input, reason } => {
+                write!(f, "invalid datetime literal {input:?}: {reason}")
+            }
+            DateTimeParseError::Unparseable(input) => write!(
+                f,
+                "unparseable datetime literal {input:?}: expected a relative duration \
+                 (e.g. \"5h\", \"-3m\"), a time-of-day (e.g. \"12:00\"), or a date/datetime \
+                 (e.g. \"2024-01-15\" or \"2024-01-15T12:00:00Z\")"
+            ),
+        }
+    }
+}
+
+impl std::error::Error for DateTimeParseError {}
+
+/// Parses a filter datetime literal into an absolute [`Timestamp`], resolving any
+/// relative or day-relative forms against `now`.
+///
+/// `now` is taken as a parameter (rather than this function calling `Timestamp::now()`
+/// internally) so callers can pass a fixed value in tests.
+pub fn parse_datetime(input: &str, now: Timestamp) -> Result<Timestamp, DateTimeParseError> {
+    let trimmed = input.trim();
+    if trimmed.is_empty() {
+        return Err(DateTimeParseError::Unparseable(input.to_string()));
+    }
+
+    if let Some(result) = try_relative(trimmed, now) {
+        return result;
+    }
+
+    if let Some(result) = try_full_datetime(trimmed) {
+        return result;
+    }
+
+    if let Some(result) = try_time_of_day(trimmed, now) {
+        return result;
+    }
+
+    Err(DateTimeParseError::Unparseable(input.to_string()))
+}
+
+/// Tries to interpret `input` as a signed relative humantime duration (`5h`, `-3m`).
+///
+/// Returns `None` if `input` doesn't look like a relative duration at all (so the
+/// caller can try the other formats); returns `Some(Err(..))` if it looks like one but
+/// the duration itself is malformed.
+fn try_relative(input: &str, now: Timestamp) -> Option<Result<Timestamp, DateTimeParseError>> {
+    let (sign, rest) = match input.strip_prefix('-') {
+        Some(rest) => (-1i128, rest),
+        None => match input.strip_prefix('+') {
+            Some(rest) => (1i128, rest),
+            None => (1i128, input),
+        },
+    };
+
+    // Only treat this as an attempted relative duration if it starts with a digit
+    // followed by (eventually) an alphabetic unit -- this keeps dates like
+    // "2024-01-15" (which start with digits but aren't durations) from being
+    // mis-attempted here and reported as "invalid duration" instead of falling
+    // through to the date parser.
+    let looks_like_duration = rest
+        .chars()
+        .next()
+        .is_some_and(|c| c.is_ascii_digit())
+        && rest.chars().any(|c| c.is_ascii_alphabetic())
+        && !rest.contains('-')
+        && !rest.contains(':');
+    if !looks_like_duration {
+        return None;
+    }
+
+    match humantime::parse_duration(rest) {
+        Ok(duration) => {
+            let nanos = duration.as_nanos() as i128 * sign;
+            let now_nanos = now.as_millis() as i128 * 1_000_000;
+            let result_millis = ((now_nanos + nanos) / 1_000_000) as i64;
+            Some(Ok(Timestamp::from_millis(result_millis)))
+        }
+        Err(err) => Some(Err(DateTimeParseError::Invalid {
+            input: input.to_string(),
+            reason: format!("not a valid relative duration: {err}"),
+        })),
+    }
+}
+
+/// Tries to interpret `input` as a full date or RFC3339-ish datetime literal.
+fn try_full_datetime(input: &str) -> Option<Result<Timestamp, DateTimeParseError>> {
+    // Only attempt this path for inputs shaped like a date (contain a '-' separating
+    // digits, e.g. "2024-01-15"); otherwise let other parsers have a shot.
+    let looks_like_date = input.len() >= 8 && input.as_bytes()[4] == b'-';
+    if !looks_like_date {
+        return None;
+    }
+
+    if let Ok(dt) = DateTime::parse_from_rfc3339(input) {
+        return Some(Ok(timestamp_from_datetime(dt.with_timezone(&Utc))));
+    }
+
+    match NaiveDate::parse_from_str(input, "%Y-%m-%d") {
+        Ok(date) => {
+            let dt = date.and_hms_opt(0, 0, 0).expect("midnight is always valid");
+            Some(Ok(timestamp_from_datetime(Utc.from_utc_datetime(&dt))))
+        }
+        Err(_) => Some(Err(DateTimeParseError::Invalid {
+            input: input.to_string(),
+            reason: "not a valid date or RFC3339 datetime (expected e.g. \"2024-01-15\" or \
+                      \"2024-01-15T12:00:00Z\")"
+                .to_string(),
+        })),
+    }
+}
+
+/// Tries to interpret `input` as a bare time-of-day (`12:00`, `00:00`, `23:59:59`),
+/// resolved against the UTC calendar day of `now`.
+fn try_time_of_day(
+    input: &str,
+    now: Timestamp,
+) -> Option<Result<Timestamp, DateTimeParseError>> {
+    if !input.contains(':') {
+        return None;
+    }
+
+    let time = NaiveTime::parse_from_str(input, "%H:%M:%S")
+        .or_else(|_| NaiveTime::parse_from_str(input, "%H:%M"));
+
+    let time = match time {
+        Ok(time) => time,
+        Err(err) => {
+            return Some(Err(DateTimeParseError::Invalid {
+                input: input.to_string(),
+                reason: format!("not a valid time-of-day (expected e.g. \"12:00\"): {err}"),
+            }));
+        }
+    };
+
+    let now_millis = now.as_millis();
+    let now_dt = Utc.timestamp_millis_opt(now_millis).single().unwrap_or_else(|| {
+        // Fall back to the epoch date if `now` is somehow out of range; this should
+        // not happen in practice since `Timestamp` always comes from a valid instant.
+        Utc.timestamp_millis_opt(0).single().expect("epoch is valid")
+    });
+    let today = now_dt.date_naive();
+    let dt = Utc.from_utc_datetime(&today.and_time(time));
+    Some(Ok(timestamp_from_datetime(dt)))
+}
+
+fn timestamp_from_datetime(dt: DateTime<Utc>) -> Timestamp {
+    Timestamp::from_millis(dt.timestamp_millis())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// A fixed "now": 2024-06-15T12:34:56Z.
+    fn fixed_now() -> Timestamp {
+        let dt = Utc.with_ymd_and_hms(2024, 6, 15, 12, 34, 56).unwrap();
+        Timestamp::from_millis(dt.timestamp_millis())
+    }
+
+    #[test]
+    fn relative_positive_duration() {
+        let now = fixed_now();
+        let got = parse_datetime("5h", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 17, 34, 56).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn relative_negative_duration() {
+        let now = fixed_now();
+        let got = parse_datetime("-3m", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 12, 31, 56).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn relative_explicit_plus() {
+        let now = fixed_now();
+        let got = parse_datetime("+10s", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 12, 35, 6).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn time_of_day_resolves_to_today() {
+        let now = fixed_now();
+        let got = parse_datetime("12:00", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 12, 0, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn time_of_day_midnight() {
+        let now = fixed_now();
+        let got = parse_datetime("00:00", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 0, 0, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn time_of_day_end_of_day() {
+        let now = fixed_now();
+        let got = parse_datetime("23:59", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 23, 59, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn time_of_day_with_seconds() {
+        let now = fixed_now();
+        let got = parse_datetime("23:59:59", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 6, 15, 23, 59, 59).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn full_date_only() {
+        let now = fixed_now();
+        let got = parse_datetime("2024-01-15", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 1, 15, 0, 0, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn full_datetime_rfc3339() {
+        let now = fixed_now();
+        let got = parse_datetime("2024-01-15T12:00:00Z", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 1, 15, 12, 0, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn full_datetime_rfc3339_with_offset() {
+        let now = fixed_now();
+        let got = parse_datetime("2024-01-15T12:00:00+02:00", now).expect("should parse");
+        let expected = Utc.with_ymd_and_hms(2024, 1, 15, 10, 0, 0).unwrap();
+        assert_eq!(got.as_millis(), expected.timestamp_millis());
+    }
+
+    #[test]
+    fn invalid_time_of_day_is_an_error() {
+        let now = fixed_now();
+        let err = parse_datetime("25:99", now).expect_err("should not parse");
+        match err {
+            DateTimeParseError::Invalid { .. } => {}
+            other => panic!("expected Invalid, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn invalid_date_is_an_error() {
+        let now = fixed_now();
+        let err = parse_datetime("2024-13-40", now).expect_err("should not parse");
+        match err {
+            DateTimeParseError::Invalid { .. } => {}
+            other => panic!("expected Invalid, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn totally_unparseable_input_is_an_error() {
+        let now = fixed_now();
+        let err = parse_datetime("not a date at all", now).expect_err("should not parse");
+        match err {
+            DateTimeParseError::Unparseable(_) => {}
+            other => panic!("expected Unparseable, got {other:?}"),
+        }
+    }
+
+    #[test]
+    fn empty_input_is_an_error() {
+        let now = fixed_now();
+        let err = parse_datetime("", now).expect_err("should not parse");
+        assert!(matches!(err, DateTimeParseError::Unparseable(_)));
+    }
+
+    #[test]
+    fn does_not_panic_on_garbage() {
+        let now = fixed_now();
+        for input in ["-", ":", "--", "::", "5", "h5", "T", "-T:", "🎉"] {
+            let _ = parse_datetime(input, now);
+        }
+    }
+}
diff --git a/src/main.rs b/src/main.rs
index 4b50470..49fb441 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -2,6 +2,7 @@ mod api;
 mod data_value;
 mod dto;
 mod engine;
+mod filter_datetime;
 mod index_key;
 mod ingest;
 mod metric_primitive;