Best way to alter Siemens timer on HMI

I have a Siemens S7300,connected to a HMI and want to be able to display and alter a timer value.

I have read a couple of suggestions but would appreciate any advice, as the Siemens "help" isn't very helpful!

Thanks

First of all, use the IEC timers, not the S5Timers.
Both the setpoint and actual values will be on TIME format. TIME is just another way of displaying a DINT. If the millisecond resolution is too finegrained, you can apply some scaling in the Tag (f.ex. PLC=1000 HMI=1 to scale to seconds).