I use Number GV and just set them to the current unix time in seconds (RM allows for setting in seconds or milliseconds). Do this twice and compare the two and you have the difference in time between the two events in seconds. Do some variable math and you have days, hours, minutes, seconds.
It's funny, I made my rules with unix time well over a year ago, the topic hasn't come up since, other than last night and today, lol. Here is my post from last night: Time Global Variable Type
I use them for some tiles on my dashboard that I want to inform me of how many days until certain filters need to be changed. Here are the tiles on my dashboard:
