Timer Jump refers to a problem with the minesweeper clock in Windows 98 and earlier versions of the operating system. The first "second" of a game is usually less than a real second because the game clock looks at the system clock.
For example, if you start a game when your computer clock is at 0.750 of a second, then the first "second" of your game will only last the remaining 0.250 seconds. When the computer clock reaches 1.000 the game clock will increment. For the rest of the game, both clocks are synchronised. As a result, players noticed that the game clock often "jumped" from 1 to 2 quickly, making it almost impossible to score 1 on Beginner. (The Windows minesweeper clock begins at 1 on your first click).
On average this means that a score made on Windows 98 or earlier is 0.500 seconds faster than the same score made on Windows XP or later.