Source: https://dl.dropboxusercontent.com/u/3658181/PiotrPiasecki-BitcoinMasterThesis.pdf
Cite:
4.5.5 Clock drift
The Standard Client exchanges the current machine time with its peers when establishing new connections. It also maintains a counter representing the med ian network time reported by the Client's peers. If the counter differs by more than 70 minutes than the Node's machine time, it is reset to represent the latter
Methinks, that "Clock drift" means the control of time shifting (to forward or to backward).