1 files changed, 63 insertions, 0 deletions
diff --git a/src/utils/datetime.test.ts b/src/utils/datetime.test.ts
new file mode 100644
index 0000000..dd239b2
--- /dev/null
+++ b/src/utils/datetime.test.ts
@@ -0,0 +1,63 @@
+import { assertEquals, assertMatch } from "@std/assert";
+import { describe, it } from "@std/testing/bdd";
+import { toIso8601Full, toIso8601FullUTC } from "./datetime.ts";
+import { FakeTime } from "@std/testing/time";
+
+describe("toIso8601Full", () => {
+  it("formats current local time with offset", () => {
+    const date = new Date();
+    const result = toIso8601Full(date);
+
+    assertMatch(
+      result,
+      /^[+-]\d{6}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}(Z|[+-]\d{2}:\d{2})$/,
+    );
+  });
+
+  it("handles dates before year 0 (BC)", () => {
+    const date = new Date(-2000, 0, 1, 0, 0, 0, 0);
+    const result = toIso8601Full(date);
+
+    assertMatch(result, /^-\d{6}-01-01T00:00:00\.000(Z|[+-]\d{2}:\d{2})$/);
+  });
+
+  it("pads components correctly", () => {
+    const date = new Date(7, 0, 2, 3, 4, 5, 6);
+    const result = toIso8601Full(date);
+
+    assertMatch(result, /^\+001907-01-02T03:04:05\.006(Z|[+-]\d{2}:\d{2})$/);
+  });
+
+  it("handles positive and negative timezone offsets", () => {
+    const date = new Date("2025-06-17T12:00:00Z");
+    const result = toIso8601Full(date);
+
+    assertMatch(
+      result,
+      /^[+-]\d{6}-06-17T\d{2}:\d{2}:\d{2}\.\d{3}(Z|[+-]\d{2}:\d{2})$/,
+    );
+  });
+});
+
+describe("toIso8601FullUTC", () => {
+  it("always formats in UTC with 'Z'", () => {
+    const date = new Date(Date.UTC(2025, 11, 31, 23, 59, 59, 999));
+    const result = toIso8601FullUTC(date);
+
+    assertEquals(result, "+002025-12-31T23:59:59.999Z");
+  });
+
+  it("pads milliseconds and components correctly", () => {
+    const date = new Date(Date.UTC(7, 0, 2, 3, 4, 5, 6));
+    const result = toIso8601FullUTC(date);
+
+    assertEquals(result, "+001907-01-02T03:04:05.006Z");
+  });
+
+  it("handles BC dates (negative years)", () => {
+    const date = new Date(Date.UTC(-44, 2, 15, 12, 0, 0, 0));
+    const result = toIso8601FullUTC(date);
+
+    assertMatch(result, /^-\d{6}-03-15T12:00:00\.000Z$/);
+  });
+});