S7 Timer 300/400

Hi
I want to be able to manipulate the value of elapsed time in S7, and I still want it to be as accurate as possible.
It seems to impossible to manipulate with the build in timers.
S_ODT (S5-timer) and IEC Timer(SFB4)(S7-Timer)
I have notice that it is possible to manipulate the STIME in the SFB4, so I have used SFB4 like below, and it looks like it works, but will I ever get a time out on the timer?
And what will happens when it flips over to a negative value? I think the math will be correct?

Who don't want a timer with a negative preset time to predict events?

Ups no immage

Do you want to measure PLC scan time or have a timer that can be adjusted?

Basic I want to have a timer who can be adjusted.
I add the scan time to my "timer" every scan.
Elsewhere I determine if time is elapsed with a compare block.
Start and reset of the timer are not shown in my code, but hopefully you still understand the principle.


Start:
NO before ADD

Reset:
MOVE 0 to "timer"

As a side benefit I get a retentive "IEC timer" if is working.
This block might be elsewhere, but I haven't be able to find it.

I would normally make a timer to be adjusted from SCADA as the attached picture.
in the example has been applied pulse from Clock Memory bit (hardware config) and the example used one second pulse.
Greater accuracy is achieved by use of 100ms pulse