s7-300 timer value

the s7-300 timer show the time left but i want to display how much time has finished like s7-200 timers
and also as in s7-200 when u want to overwrite the timer PV value it is accepted straight away,eg if the timer PV value is set 20 secs and if CV is 15 secs we can update the timer to 30secs,and now the done will be at new setpoint,how is this possible with s7-300 timers.

How much time has finished?
You have to load the preset value and the current time and do a math subtract operation using memory parameters.

You can use a memory to set a preset value and change it via HMI device if you are going to do it that way.

Here is the Statement List Code for it that is easier to show. Transfer it to ladder by dividing into networks if you want the ladder program.

MW 10 – Use HMI device to show the finished time
MW 20- Change the time via HMI device
Program assume that you use a pulse timer

Program

A I 1.2
FR T1// enable timer
A I 1.3
L MW 10// MW 10 into Accumulator
SI T1// Pulse timer
L T1
T MW 30// Timer value transfer into MW 30
L MW 10
L MW 30
-I
T MW 20// How much time has finished

Note- If you want to see the how much time has finished, connect it to MW 20 as a outfield, and if you want to change the preset value connect it to MW 10 as a infield via the HMI device. Make sure that you give an initial value to MW 10 in OB 100, OB 101 at the starting of the device.

MW 10 – Bits 0 to 11 (12 bits) timer value in BCD code, bits 12 and 13 (two bits) time base