Farming game Time management

I am making farming game and I dont know how to make time related system.

What I want is first, plant seed and second, after few hour that I set plant to be available to harvest. Probably player wouldn't wait few hour to harvest after planting. They would turn off my program. I want my program to count how much time has passed since they planted if program is off or not. I am pretty confused with this time design.

What I only know is using time.deltatime to count but this wouldn't work if program is off. Is there stuff that I can use or learn? please tell me.