Freddie wrote:

I record meteorological data using a 0900-0900 day. If you record a minimum
temperature of, say, 3.5C at 03:30 on 15 January, to which date is that
temperature attributed? Is it the 14th (the date on which the recording
period started) or the 15th (the date on which the temperature was actually
recorded)?

No matter what time the min actually occurs it is attributed to the day on
which the 24-hour recording period ends. In the example you give, it would be
attributed to the 15th.

In Tideswell the min in the 24-hour period ending at 0900z today actually
occurred at 0900z yesterday. Nevertheless, it is attributed to today.

With max temperatures it's different. The maximum during the 0900-0900 period
is attributed to the day on which the recording period starts. In Tideswell the
max in the 24-hour period ending 0900z today occurred at 0325z this morning.
That goes down as the max for yesterday.

These conventions stem from the age before electronic recording instruments.
Max/min temperatures were all manual readings and, at least in the UK, 0900z
became the start/end of the meteorological day.

--
Norman Lynagh
Tideswell, Derbyshire
303m a.s.l.