input: Write a python function to calculate the gap in days between two dates.
  assert days_gap("01/01/2021", "01/31/2021") == 30
  assert days_gap("12/01/2020", "12/01/2021") == 365
  assert days_gap("03/15/2021", "02/15/2021") == 28
output: def days_gap(date1, date2):
      from datetime import datetime
      date_format = "%m/%d/%Y"
      d1 = datetime.strptime(date1, date_format)
      d2 = datetime.strptime(date2, date_format)
      return abs((d2 - d1).days)