(df2.groupby('MEMSHPNUM')['ISSDAT'].diff() / np.timedelta64(1, 'D')) in days code example

Example: timedelta64 total_mins

import datetime


def minutes(date_time_older, date_time_newer):
  date_time_difference = round((date_time_newer-date_time_older).total_seconds()/60)
  return date_time_difference