Note that any of these are only accurate if the start times are in UT1. If it is 'localtime' then the results will be incorrect and that conversion from 'localtime' to ut1 via the 'utc' operand in the datetime functions have varying accuracy depending on the Operating System -- that is to say that the conversion will be accurate for 'now' but not necessarily in the past, nor at the end of the year (depending on the whim of pollitians between now and then)