If you are experiencing issues with the timers in your PLC programs which were previously working you may simply need to restart the PC.
Why would this resolve the problem? Well, the 'Timers' part of the code uses the inbuilt Microsoft Windows function called GetTickCount, which has a known issue where if a PC is not restarted for more than 25 days (but less than 49.7 days) then the function does not work correctly. The solution is to simply reboot the PC and hence reset the internal counter.
Tip - One way of identifying how long your PC has been running is by running 'Task Manager' (CTRL+SHIFT+ESC) clicking on the 'Performance' tab and then selecting 'CPU'. The 'Up Time' is displayed at the bottom of the screen.
If this doesn't resolve your specific issue please raise a support ticket