import datetime

def get_diff_days(pdate, days):
	pdate_obj = datetime.datetime.strptime(pdate, '%Y-%m-%d')
	time_gap = datetime.timedelta(days=days)
	pdate_result = pdate_obj - time_gap
	return pdate_result.strftime("%Y-%m-%d")

print(get_diff_days("2021-04-28", 1))
print(get_diff_days("2021-04-28", 3))
print(get_diff_days("2021-04-28", 7))
print(get_diff_days("2021-04-01", 3))