OpenEVSE was written for and has only been tested on 16 x 2 displays. You will need to rewrite the LCD libraries and LCD display code to support 20 x 4.
The elapsed time counter was only shown when Ammeter was disabled.
This may be what you are looking for...
In the OpenEVSE 5x version, the Charging state displayed the duration of the car charging. I have the 6x version and instead of time I have kW and kWh. I inserted the 20x4 display - I changed the settings and wants to add time to line 3. What should I do?