just to add to the examples here: last week i was writing code to handle calibration data from seismic detectors. these are connected to GPS receivers and so have pretty accurate times. yet when i triggered a calibration it would start 1 minute in the past. somehow the receiver was moving back in time before starting the calibration....
...or ntpd on the test computer had failed and the machine was a minute fast :o) so the code to search for data now looks backwards in time as well as forwards.
incidentally, does anyone know of a really good API for time (including calendars etc)? python's (which is largely a thin layer over C) is a horrible mess, for example.