Computers usually tell time by counting the number of seconds since a certain “epoch” time. Then, before displaying the time to you, they do all of the crazy math that defines days of the year and leap years, and even time zones daylight savings time. On Linux systems, the “epoch” is midnight on January 1st, 1970.
At any time, you can tell how many seconds have passed since the epoch by typing:
Tonight, at 6:31:30 pm local time, we reached a magic moment when the date was exactly 1234567890 seconds since epoch.
I took this opportunity to show the kids how computers keep track of the time, and to explain time zones (even though they are comfortable with the fact that is it morning in Malaysia when it’s evening here, they did not know about time zones). At the magic moment, we took time out from our pizza supper to watch the time change on my laptop.