I’d write a Rule that triggers when the Switch turns ON.
Create a Number Item to store the Number of minutes and reset this Item to 0 when the Rule triggers.
In the Rule
MyNumberItem.postUpdate(0)
where MyNumberItem is the name of the Number Item you created.
Create a Design Pattern: Looping Timers that runs every minute and adds one to the Number Item.
Read the design pattern and come back with specific questions.
Create a JS transformation that converts the number of minutes to HH:MM format to be used in the Item’s label.
https://www.openhab.org/addons/transformations/javascript/
When the Switch turns OFF, cancel the looping timer.
See the design pattern