List Contacts between last_updated date range

When calling rest end point for list contacts and applying entries for last_updated date/time range what comparison is used for date/time comparison.

Example, is it:
return list for since >= since_datetime_value && until < until_datetime_value, or
return list for since > since_datetime_value && until <= until_datetime_value, or
return list for since >= since_datetime_value && until <= until_datetime_value, etc…

Thanks

Bump. Any feedback?