strtotime('now')	2025/01/18 14:13:08
strtotime('+2 day')	2025/01/20 14:13:08
strtotime('2 day')	2025/01/20 14:13:08
strtotime('-2 day')	2025/01/16 14:13:08
strtotime('2015/08/01')	2015/08/01 00:00:00
strtotime('2039/08/01')	2039/08/01 00:00:00
strtotime('yesterday')	2025/01/17 00:00:00
strtotime('tomorrow')	2025/01/19 00:00:00
strtotime('yesterday 09:00:00')	2025/01/17 09:00:00
strtotime('+3 week')	2025/02/08 14:13:08
strtotime('-3 week')	2024/12/28 14:13:08
strtotime('next sunday')	2025/01/19 00:00:00
strtotime('next sun')	2025/01/19 00:00:00
strtotime('last sunday')	2025/01/12 00:00:00
strtotime('+4 sunday')	2025/02/09 14:13:08
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')	2025/03/18 14:13:08
strtotime('-2 month')	2024/11/18 14:13:08
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')	2025/02/28 14:13:08
strtotime('+1 year')	2026/01/18 14:13:08
strtotime('-1 year')	2024/01/18 14:13:08
strtotime('-1 year 13:23:33')	2024/01/18 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')	2025/01/18 16:13:08
strtotime('+4 min')	2025/01/18 14:17:08
strtotime('+6 sec')	2025/01/18 14:13:14
strtotime('now') - strtotime('today')...今日の 0時からの秒数	51188
strtotime('now') - strtotime('today 10:00:00')...今日の10時からの秒数	15188