input: Write a python function to calculate the difference in days between two dates.
  assert date_diff("01/01/2020", "01/02/2020") == 31
  assert date_diff("31/12/2020", "01/01/2021") == 1
  assert date_diff("28/02/2020", "01/03/2020") == 2
output: from datetime import datetime
  
  def date_diff(date1, date2):
      format_str = "%d/%m/%Y"  # The format
      start_date = datetime.strptime(date1, format_str)
      end_date = datetime.strptime(date2, format_str)
      delta = end_date - start_date
      return abs(delta.days)