Will the victim know it is a prank right away?

Almost never. A malfunctioning system clock is a known (if uncommon) computer problem, so the victim will assume it is a legitimate hardware or software issue. They will try syncing the time, restarting the computer, checking BIOS settings, and searching for solutions online -- all while the clock keeps jumping to random times every 30 seconds. The prank is especially effective because the problem looks completely real.