Recently it looks like the method changed where mIRC now does interval*1000 if you don't use -m or -h and use that as millisecond anyway. Unfortunately the check on the parameter is now incorrect, it allows the value to overflow, triggering the command immediately:
//timertest 1 2147483 echo -a ok | timers
* Timer test 1 time(s) 2147483000ms delay echo -a ok
//timertest 1 2147484 echo -a ok | timers
* Timer test 1 time(s) -2147483296ms delay echo -a ok
-
ok
-
where 2147484*1000=2147484000 which exceeds the maximum value of an int 32bits (2147483647).
