strtotime() function can be used for date arithmetic. Following are some examples:
Get next date
print date('Y-m-d', strtotime('+1 day', strtotime('2013-09-30')));
Prints
2013-10-01
Get previous month
print date('Y-m-d', strtotime('-1 month', strtotime('2013-09-30')));
Prints
2013-08-30
Go back 5 years
print date('Y-m-d', strtotime('-5 year', strtotime('2013-09-30')));
Prints
2013-04-30