In addition to using WALLCLOCK time as Ron mentioned, you can also create your own free running timer in RSLogix5000 by just creating an unconditional timer and using the .ACC bits of the timer. Never reset this timer, it will never reach its preset.
Free_Running_Clock.30
--------(OTU)-----------------+-TON---------------------+-(EN)
|Timer Free_Running_Clock +-(DN)
|PRE 2000000000|
|ACC 0|
+-------------------------+
Unconditionally unlatch Free_Running_Clock.ACC.30 so that the timer will never time out. It will count to 1073741.824 seconds and then start over.
Now you can access each bit in the ACC word, with each bit behaving similar to the S:4, except now you have 29 bits available for use instead of 16.
The bit on time is 2
Y milliseconds where Y is the bit number 0-29. The duty cycle is 2 times the bit on time, just like in S:4.
Free_Running_Timer.ACC.0 = 1mS (2mS duty cycle)
Free_Running_Timer.ACC.1 = 2mS
Free_Running_Timer.ACC.2 = 4ms
etc.