strtotime('now')	2025/12/25 06:24:18
strtotime('+2 day')	2025/12/27 06:24:18
strtotime('2 day')	2025/12/27 06:24:18
strtotime('-2 day')	2025/12/23 06:24:18
strtotime('2015/08/01')	2015/08/01 00:00:00
strtotime('2039/08/01')	2039/08/01 00:00:00
strtotime('yesterday')	2025/12/24 00:00:00
strtotime('tomorrow')	2025/12/26 00:00:00
strtotime('yesterday 09:00:00')	2025/12/24 09:00:00
strtotime('+3 week')	2026/01/15 06:24:18
strtotime('-3 week')	2025/12/04 06:24:18
strtotime('next sunday')	2025/12/28 00:00:00
strtotime('next sun')	2025/12/28 00:00:00
strtotime('last sunday')	2025/12/21 00:00:00
strtotime('+4 sunday')	2026/01/18 06:24:18
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')	2026/02/25 06:24:18
strtotime('-2 month')	2025/10/25 06:24:18
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')	2026/01/31 06:24:18
strtotime('+1 year')	2026/12/25 06:24:18
strtotime('-1 year')	2024/12/25 06:24:18
strtotime('-1 year 13:23:33')	2024/12/25 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')	2025/12/25 08:24:18
strtotime('+4 min')	2025/12/25 06:28:18
strtotime('+6 sec')	2025/12/25 06:24:24
strtotime('now') - strtotime('today')...今日の 0時からの秒数	23058
strtotime('now') - strtotime('today 10:00:00')...今日の10時からの秒数	-12942