input: Write a python function to calculate the number of days between two dates.
  assert calculate_days_between_dates('2020-01-01', '2020-01-30') == 29
  assert calculate_days_between_dates('2021-06-15', '2021-06-20') == 5
  assert calculate_days_between_dates('2019-12-31', '2020-01-01') == 1
output: from datetime import datetime
  
  def calculate_days_between_dates(date1, date2):
      date_format = "%Y-%m-%d"
      d1 = datetime.strptime(date1, date_format)
      d2 = datetime.strptime(date2, date_format)
      delta = d2 - d1
      return delta.days