broofa/node-uuid — comment on commit 42285edef9cadfd39d9b01b395ea466e84a4ffde
File: uuid.js line 144
By broofa:

Calculating the time difference by first subtracting the components from one another solves the precision problem. If the two times are < 2^^52 X 100ns apart we have enough precision here, and if the times are further apart than that, the degree of precision won't matter anyway.