eg date("Y-m-d", mktime()+($days * 86400)) will return a date $days-1 in the future if it runs on leap second day. No it won't. Those functions deal in UNIX-e

eg date("Y-m-d", mktime()+($days * 86400)) will return a date $days-1 in the future if it runs on leap second day. No it won't. Those functions deal in UNIX-epoch times, which ignore leap seconds.