If your minimum temperature is for the period 18 to 06 and max for 06 to 18
the only difference you will notice on switching to 21 hrs is that the mean min
will be slightly lower and the mean max slightly higher, due to including those
minima and maxima that occur between 18 and 21 hrs. There aren't many but
they're not unheard-of. I doubt if the difference over a long period would
amount to more than 0.2°C, almost all of it occurring in winter. My own
practice is day-max night-min, observation hours for this being 08 and 20 hrs
clock time. If you do 24-hr max and min you will notice no difference at all,
naturally, if the morning observation time is unchanged.

Best wishes, Tudor Hughes, Warlingham, Surrey.