Creating a DateTimeUtc schema without milliseconds and the 'Z' in the string format can be done m...

const TimestampToTimestampWithoutMillis = S.transformOrFail(
  S.String,
  S.String.pipe(S.pattern(/^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3}Z$/)),
  {
    encode: (withMillis) =>
      Effect.gen(function* () {
        const trimmed = withMillis.split(".")[0]
        if (trimmed === undefined) {
          return yield* ParseResult.fail(
            new ParseResult.Unexpected(
              withMillis,
              "Invalid date format, expected ISO 8601 with milliseconds"
            )
          )
        }
        return trimmed
      }),
    decode: (s) => Effect.succeed(s)
  }
)

const DateTimeUtcWithoutMillis = TimestampToTimestampWithoutMillis.pipe(S.compose(S.DateTimeUtc))