Start: 2026-08-31 00:00:00
modify('+1 month'): 2026-10-01 00:00:00
add(DateInterval('P1M')): 2026-10-01 00:00:00
modify('first day of next month'): 2026-09-01 00:00:00
modify('first day of this month')->modify('+1 month'): 2026-09-01 00:00:00