Fix DateTime.makeUnsafe incorrectly appending "Z" to date strings containing "GMT" (#1781)

Author: gcanti

.changeset/fix-datetime-gmt.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix `DateTime.makeUnsafe` incorrectly appending "Z" to date strings containing "GMT"

packages/effect/src/internal/dateTime.ts

@@ -232,7 +232,14 @@ export const makeUnsafe = <A extends DateTime.DateTime.Input>(input: A): DateTim
   return fromDateUnsafe(new Date(input)) as DateTime.DateTime.PreserveZone<A>
 }
 
-const hasZone = (input: string): boolean => /Z|[+-]\d{2}$|[+-]\d{2}:?\d{2}$|\]$/.test(input)
+/**
+ * Detects whether a date string already contains timezone info.
+ * Without a zone, `new Date("2024-01-01T12:00:00")` is parsed as local time,
+ * so `makeUnsafe` appends "Z" to force UTC interpretation.
+ * This check prevents appending "Z" to strings that already have a zone
+ * (e.g. "2024-01-01T12:00:00Z", "...+05:30", "...GMT"), which would produce invalid dates.
+ */
+const hasZone = (input: string): boolean => /Z|GMT|[+-]\d{2}$|[+-]\d{2}:?\d{2}$|\]$/.test(input)
 
 const minEpochMillis = -8640000000000000 + (12 * 60 * 60 * 1000)
 const maxEpochMillis = 8640000000000000 - (14 * 60 * 60 * 1000)

packages/effect/test/DateTime.test.ts

@@ -457,12 +457,24 @@ describe("DateTime", () => {
 
   describe("makeUnsafe", () => {
     it("treats strings without zone info as UTC", () => {
-      let dt = DateTime.makeUnsafe("2024-01-01 01:00:00")
+      const dt = DateTime.makeUnsafe("2024-01-01 01:00:00")
       strictEqual(dt.toJSON(), "2024-01-01T01:00:00.000Z")
+    })
+
+    it("does not append Z to strings ending with Z", () => {
+      const dt = DateTime.makeUnsafe("2026-01-27T17:14:06.000Z")
+      strictEqual(dt.toJSON(), "2026-01-27T17:14:06.000Z")
+    })
 
-      dt = DateTime.makeUnsafe("2020-02-01T11:17:00+1100")
+    it("does not append Z to strings with explicit offset", () => {
+      const dt = DateTime.makeUnsafe("2020-02-01T11:17:00+1100")
       strictEqual(dt.toJSON(), "2020-02-01T00:17:00.000Z")
     })
+
+    it("does not append Z to strings containing GMT", () => {
+      const dt = DateTime.makeUnsafe("Tue, 27 Jan 2026 17:14:06 GMT")
+      strictEqual(dt.toJSON(), "2026-01-27T17:14:06.000Z")
+    })
   })
 
   describe("Disambiguation", () => {

packages/effect/test/schema/SchemaGetter.test.ts

@@ -1,4 +1,4 @@
-import { Effect, Option, Result, SchemaGetter } from "effect"
+import { DateTime, Effect, Option, Result, SchemaGetter } from "effect"
 import { describe, it } from "vitest"
 import { assertSome, deepStrictEqual } from "../utils/assert.ts"
 
@@ -21,6 +21,14 @@ describe("SchemaGetter", () => {
     assertSome(result, 2)
   })
 
+  it("dateTimeUtcFromInput", async () => {
+    const decoding = makeAsserts(SchemaGetter.dateTimeUtcFromInput<string>())
+    await decoding("2024-01-01 01:00:00", DateTime.makeUnsafe("2024-01-01T01:00:00.000Z"))
+    await decoding("2020-02-01T11:17:00+1100", DateTime.makeUnsafe("2020-02-01T00:17:00.000Z"))
+    // should support strings with explicit GMT zone
+    await decoding("Tue, 27 Jan 2026 17:14:06 GMT", DateTime.makeUnsafe("2026-01-27T17:14:06.000Z"))
+  })
+
   describe("decodeFormData / encodeFormData", () => {
     const decoding = makeAsserts(SchemaGetter.decodeFormData())
     const encoding = makeAsserts(SchemaGetter.encodeFormData())