If your starting point is either February 10th, 2021 or October 2nd, 2021 at 5:08pm, and you’re trying to get to June 15th, 2021 at 3:30 pm, then you need to use a few DATEADDs to add time (or subtract time).
If you’re just trying to get 02/10/2021 5:08 pm into the same format as 2021-06-15T15:30:00.000Z but not the same date, then you could probably write a formula that looks like this:
{Date Field}&""
^ which forces a date to become a string, or build it yourself using DATETIME_FORMAT()
