Allow sub milliseconds for `DateTime`

[n1ru4l]

Is your feature request related to a problem? Please describe.

RFC 3993 supports nano seconds.

The serialize method of DateTime always returns a Date object.
The problem with this is that the Date implementation in Node.js drops nano seconds.

E.g. a date string of 2025-06-04T09:46:50.423231501+00:00 will become 2025-06-04T09:46:50.423+00:00

- 2025-06-04T09:46:50.423231501+00:00
+ 2025-06-04T09:46:50.423+00:00

Describe the solution you'd like

Using the DateTime scalar should not remove nano seconds. This would be a breaking change as the scalar seralize/parse output must be a string and can no longer be a Date instance.

Describe alternatives you've considered

Create my own DateTime64 scalar inspired by clickhouse

Additional context

• Internal Discussion