Why does the clock on my computer keep gaining time?

You're not, by any chance, traveling near the speed of light, are you?

(Sorry; I couldn't resist the physics joke.)

Seriously: About the only thing I know of that might cause a real-time clock malfunction is a failing CMOS battery. (Desktops and laptops both have them. This is a round, coin-sized cell, not the square laptop battery pack.) Is your computer more than about four years old? Could be it.

Easily replaced in a desktop. In a laptop, might be easy, might be hard.

My clock gains 24 hours every day.. JUST KIDDING! (but it really does)

You may want to check your battery on the motherboard.