Hi, welcome to the forum.
The millis() function can be used to blink a led, to calculate how long something takes and so on.
When done right, it will work perfectly fine, even during a rollover (when millis goes from 0xFFFFFFFF to 0x00000000).
If your garage door is not opened for months and you want to measure how long it has not been used, then you add a Real Time Clock (RTC) and calculate the time in days (or in hours or seconds or so).
The Blink Without Delay is the introduction to millis().