alphagov/fabric-scripts — pull request #9 by alext (2013-09-13 10:20:54+00:00)
Title: When resyncing the time use the -B option to ntpdate

From the man page:

Force the time to always be slewed using the adjtime() system call, even
if the measured offset is greater than +-128 ms. The default is to step
the time using settimeofday() if the offset is greater than +-128 ms.
Note that, if the offset is much greater than +-128 ms in this case, that
it can take a long time (hours) to slew the clock to the correct value.
During this time, the host should not be used to synchronize clients.

TL;TR this will cause the kernel to gradually adjust the time over several minutes. This will avoid issues caused by sudden steps in the time.