Preserve DateTime microsecond precision

tests/test_datetime_postgres.nim

@@ -0,0 +1,30 @@
+import std/[os, times, unittest]
+import db_connector/[db_common, db_postgres]
+
+import fiber_orm/util
+
+proc utcDateTime(nanosecond: NanosecondRange): DateTime =
+  dateTime(
+    2026, mAug, 26, 14, 32, 10, nanosecond,
+    utc())
+
+let connectionString = getEnv("FIBER_ORM_TEST_DB")
+if connectionString.len == 0:
+  quit "FIBER_ORM_TEST_DB must contain a PostgreSQL connection string"
+
+suite "PostgreSQL DateTime round trips":
+  test "preserves whole-second, millisecond, and microsecond values":
+    let db = db_postgres.open("", "", "", connectionString)
+    defer: db.close()
+    db.exec(sql"SET TIME ZONE 'UTC'")
+
+    for expected in [
+      utcDateTime(0),
+      utcDateTime(123_000_000),
+      utcDateTime(123_457_000),
+    ]:
+      let dbValue = db.getValue(
+        sql"SELECT ?::timestamp with time zone",
+        dbFormat(expected))
+      let actual = parsePGDatetime(dbValue)
+      check actual.toTime == expected.toTime