For many years figures from Met Office sites were based on 0900-2100 max and
2100-0900 min (these are still calculated from NCM reports) precisely to
avoid singleton cold nights counting twice. Average differences between
monthly averages worked out from 09-21/21-09 stations and 0909 stations were
as much as 0.7C for December minima, though depending on the character of
the month the figure could be even higher.

The standardisation on 0909 was presumably to make it easy for once a day
stations, though there is no ideal time of day for a reset that would be
valid all year.

I think it is a great limitation of most (if not all) AWS weather software
interfaces that you are stuck with a non-standardised 00-24 daily cycle,
though perhaps that means it is the new standard....

Stuart