[v6][auth] UserMetadata times are milliseconds, should be ISO Date

[Ehesp]

@Ehesp @Salakar

[stale]

Hello 👋, to help manage issues we automatically close stale issues.
This issue has been automatically marked as stale because it has not had activity for quite some time. Has this issue been fixed, or does it still require the community's attention?

This issue will be closed in 15 days if no further activity occurs.
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