Problem in timer

Hey guys am using Allenbradley Micrologix 1200. Programming it by RSlogix500. in a program i use to take the time and do mathematical calculations. while using like that i got a problem i will reset the counter when it reaches 40mS and again it timer runs for the first 3 times i will be getting untoggled value but in the fourth time my timer value will be toggled to 39. i don't understand why? while i use free running clock also this problem araissed. i think this is occurring by scanning process.. how to rectify this

Because of scan time....The Timer is evaluated each scan..at 40ms, it's very likely that such evaluation is not going to happen at precisely 40ms, especially when you're talking about a 1ms difference.

A PLC timer is a computer instruction that operates on a location in memory. It is not a device. Since it is a computer instruction its precision is subject to when the computer instruction is executed.