diff --git a/src/Core.FSharp.ZetaId/Codec.fs b/src/Core.FSharp.ZetaId/Codec.fs
index 1b474ba84c..133b7342f7 100644
--- a/src/Core.FSharp.ZetaId/Codec.fs
+++ b/src/Core.FSharp.ZetaId/Codec.fs
@@ -10,8 +10,10 @@ module ZetaIdCodec =
     let private layout = BitLayout.Default
 
     /// Timestamp is packed into a 48-bit field; valid range [0, 2^48 - 1].
-    [<Literal>]
-    let private MaxTimestamp = 281474976710655L  // (1L <<< 48) - 1L
+    /// Typed with the `ms` measure to compose with ZetaObservation.Timestamp
+    /// (per Mika V9.3). `[<Literal>]` removed — measure-typed values can't be
+    /// F# literals; this is a regular let-binding (computed once per module init).
+    let private MaxTimestamp : int64<ms> = 281474976710655L<ms>  // (1L <<< 48) - 1L
 
     let private setBits (id: System.UInt128) (field: BitField) (fieldValue: uint64) : System.UInt128 =
         let mask = (System.UInt128.One <<< field.Width) - System.UInt128.One
@@ -37,10 +39,10 @@ module ZetaIdCodec =
         if isNull (box env) then
             raise (System.ArgumentNullException("env"))
 
-        if obs.Timestamp < 0L || obs.Timestamp > MaxTimestamp then
+        if obs.Timestamp < 0L<ms> || obs.Timestamp > MaxTimestamp then
             raise (System.ArgumentOutOfRangeException(
                 "obs", obs.Timestamp :> obj,
-                sprintf "ZetaObservation.Timestamp must be 0..%d (48-bit field). Values outside this range would silently truncate and collide." MaxTimestamp))
+                sprintf "ZetaObservation.Timestamp must be 0..%d ms (48-bit field). Values outside this range would silently truncate and collide." (int64 MaxTimestamp)))
 
         validateEnumField (byte obs.Version)    5 "Version"
         validateEnumField (byte obs.Chromosome) 5 "Chromosome"
@@ -82,7 +84,7 @@ module ZetaIdCodec =
 
         let mutable id = System.UInt128.Zero
         id <- setBits id layout.Version    (uint64 (byte obs.Version))
-        id <- setBits id layout.Timestamp  (uint64 obs.Timestamp)
+        id <- setBits id layout.Timestamp  (uint64 (int64 obs.Timestamp))
         id <- setBits id layout.Chromosome (uint64 (byte obs.Chromosome))
         id <- setBits id layout.Category   (uint64 (byte obs.Category))
         id <- setBits id layout.Firefly    (uint64 (byte obs.Firefly))
@@ -103,7 +105,7 @@ module ZetaIdCodec =
         let toByte field = byte (getBits id field)
         {
             Version    = LanguagePrimitives.EnumOfValue<byte, IdVersion>   (toByte layout.Version)
-            Timestamp  = int64 (getBits id layout.Timestamp)
+            Timestamp  = LanguagePrimitives.Int64WithMeasure<ms> (int64 (getBits id layout.Timestamp))
             Chromosome = LanguagePrimitives.EnumOfValue<byte, Chromosome>  (toByte layout.Chromosome)
             Category   = LanguagePrimitives.EnumOfValue<byte, Category>    (toByte layout.Category)
             Firefly    = LanguagePrimitives.EnumOfValue<byte, Firefly>     (toByte layout.Firefly)
diff --git a/src/Core.FSharp.ZetaId/Types.fs b/src/Core.FSharp.ZetaId/Types.fs
index 0f6dd8c2f4..d6261f3596 100644
--- a/src/Core.FSharp.ZetaId/Types.fs
+++ b/src/Core.FSharp.ZetaId/Types.fs
@@ -1,5 +1,11 @@
 namespace Zeta.Core.FSharp.ZetaId
 
+/// Milliseconds units of measure. Duplicated locally (not imported from
+/// `src/Core/Units.fs`) to preserve the zero-external-dependencies discipline
+/// of the production library. Per Mika V9.3 substrate (2026-05-21).
+[<Measure>]
+type ms
+
 /// Version field — 5 bits. Currently V1 only.
 /// Mirrors `src/Core.CSharp.ZetaId/IdVersion.cs`.
 type IdVersion =
@@ -167,7 +173,7 @@ module Momentum =
 /// Observation record. All fields explicit; no default-init shortcuts.
 type ZetaObservation = {
     Version: IdVersion
-    Timestamp: int64
+    Timestamp: int64<ms>
     Chromosome: Chromosome
     Category: Category
     Firefly: Firefly
diff --git a/tests/Tests.FSharp/ZetaId/CrossVerifyTests.fs b/tests/Tests.FSharp/ZetaId/CrossVerifyTests.fs
index e2ccb547e1..0b2c501891 100644
--- a/tests/Tests.FSharp/ZetaId/CrossVerifyTests.fs
+++ b/tests/Tests.FSharp/ZetaId/CrossVerifyTests.fs
@@ -70,7 +70,7 @@ let private toMomentum (v: FlatVector) : Momentum =
 let private toObservation (v: FlatVector) : ZetaObservation =
     {
         Version    = LanguagePrimitives.EnumOfValue<byte, IdVersion>   (checkByte v.Version "Version")
-        Timestamp  = v.Timestamp
+        Timestamp  = LanguagePrimitives.Int64WithMeasure<ms> v.Timestamp
         Chromosome = LanguagePrimitives.EnumOfValue<byte, Chromosome>  (checkByte v.Chromosome "Chromosome")
         Category   = LanguagePrimitives.EnumOfValue<byte, Category>    (checkByte v.Category "Category")
         Firefly    = LanguagePrimitives.EnumOfValue<byte, Firefly>     (checkByte v.Firefly "Firefly")