Javascript - calculate time passed while offline? (plus event occurring at specific time)

Currently, I'm writing a small program for my own personal use. So far, everything is working as expected. However, a specific problem is arising that I have no idea how to solve. I need to accomplish three things: (1) find the day a specific button is pressed and the program is started, (2) how many days have passed from now until then, and (3) know this even if the program itself has been closed in the meantime. The first two seem irritating, as I must deal with this bizarre variable which counts up in milliseconds the time right now based on midnight of January 1st, 1970, but not impossible if I were to work on it for some time. The third is where the real problem lies - I would imagine I have to save the information in the form of a cookie but I haven't the slightest idea how to do that or what resources would be needed. I tried to look it up but I couldn't quite figure out what I needed to do - it seemed like I was missing some piece of information I would need.

In addition to this, if there's a simpler way to calculate the date, that information would also be very helpful. Thanks!