[lelandbatey]

Eh, doing the math[0], it seems like you'll actually drift about 1 nanosecond every ~35 minutes if I pick a mountain near my house vs sea level. That means you'd have about 1 microsecond of difference in your clocks in about 3 weeks. For a 1 millisecond drift though, that'd take about 66 years. That's still pretty safe, but it's definitely not "you never have to worry about this˟" territory. I'm sure some others can chime in with cases where 1 microsecond clock drift (in cases expecting very accurate clocks) can cause problems.

    >>> g = 9.8 c = 299_792_458
    >>> h = 4392 # height of Mt. Rainier
    >>> drift_per_second = (g*h)/(c**2)
    >>> print(drift_per_second)
    4.789023865263744e-13
    >>> nanosecond = 1.0e-9
    >>> seconds_to_drift_one_ns = nanosecond / drift_per_second
    >>> print(seconds_to_drift_one_ns)
    2088.1081993625176
    >>> print(seconds_to_drift_one_ns / 60) # minutes till a 1 ns drift accumulates
    34.80180332270863
    >>> print((seconds_to_drift_one_ns * 1000) / 60 / 60 / 24) # days till ~1 microsecond accumulates
    24.167918974103213

[0] - http://www.leapsecond.com/great2016a/#%3A~%3Atext%3DConceptu...