No, it’s best to determine time by using number of seconds since 1970. That way you’re not having to convert months and days and check for leap year and all kinds of other crazy things.
What I’m hoping to do is keep a timer running in the app and save the timer value every second. So if you close the app and then re-open it a while later, you can compare the “seconds since 1970” to the saved value from when the app was closed and you’ll be able to tell how much time elapsed between closing and re-opening the app.