strtotime('now')	2026/03/25 07:18:51
strtotime('+2 day')	2026/03/27 07:18:51
strtotime('2 day')	2026/03/27 07:18:51
strtotime('-2 day')	2026/03/23 07:18:51
strtotime('2015/08/01')	2015/08/01 00:00:00
strtotime('2039/08/01')	2039/08/01 00:00:00
strtotime('yesterday')	2026/03/24 00:00:00
strtotime('tomorrow')	2026/03/26 00:00:00
strtotime('yesterday 09:00:00')	2026/03/24 09:00:00
strtotime('+3 week')	2026/04/15 07:18:51
strtotime('-3 week')	2026/03/04 07:18:51
strtotime('next sunday')	2026/03/29 00:00:00
strtotime('next sun')	2026/03/29 00:00:00
strtotime('last sunday')	2026/03/22 00:00:00
strtotime('+4 sunday')	2026/04/19 07:18:51
strtotime('first sun of 2015-08')	2015/08/02 00:00:00
strtotime('last sun of 2015-08')	2015/08/30 00:00:00
strtotime('last sun of 2039-08')	2039/08/28 00:00:00
strtotime('+2 month')	2026/05/25 07:18:51
strtotime('-2 month')	2026/01/25 07:18:51
strtotime('2015/08/01 +1 month')	2015/09/01 00:00:00
strtotime('+1 month', strtotime('2015/08/01'))	2015/09/01 00:00:00
strtotime('2015/08/31 +1 month')	2015/10/01 00:00:00
strtotime('2015-08 +1 month')	2015/09/01 00:00:00
strtotime('2039-08 +1 month')	2039/09/01 00:00:00
strtotime('first day of 2015/08/12')	2015/08/01 00:00:00
strtotime('first day of', strtotime('2015/08/12'))	2015/08/01 00:00:00
strtotime('last day of 2015/08/12')	2015/08/31 00:00:00
strtotime('last day of next month')	2026/04/30 07:18:51
strtotime('+1 year')	2027/03/25 07:18:51
strtotime('-1 year')	2025/03/25 07:18:51
strtotime('-1 year 13:23:33')	2025/03/25 13:23:33
strtotime('2015/01/01 -1 year')	2014/01/01 00:00:00
strtotime('-1 year', strtotime('2015/01/01'))	2014/01/01 00:00:00
strtotime('+2 hour')	2026/03/25 09:18:51
strtotime('+4 min')	2026/03/25 07:22:51
strtotime('+6 sec')	2026/03/25 07:18:57
strtotime('now') - strtotime('today')...今日の 0時からの秒数	26331
strtotime('now') - strtotime('today 10:00:00')...今日の10時からの秒数	-9669