It would be helpful for monitoring the health of a competition instance to see how many uploaders joined the contest each day. (This is different from newbie counting)
The numbers given for the UK at https://tools.wmflabs.org/wikiloves/monuments/2018/United%20Kingdom don't match the actual daily uploads which we categorise as they happen in Commons cats Category:WLM-UK 2018 unfiltered 09-xx where xx is the day of the month. You can see the numbers in those cats here: https://commons.wikimedia.org/wiki/Category:Images_from_Wiki_Loves_Monuments_2018_in_the_United_Kingdom.
I suspect the tool may be incorrectly working to UTC rather than the local midnight-to-midnight times.
@MichaelMaggs The tool does not take local time into account (except for calculating the begin- and endtime), and can only be considered an approximation. This is why several countries have uploads on 31 August. It is not designed as the 'truth' but rather as a way to analyze and monitor how the competition is progressing. It may deal differently with deletions and several other edge cases.
Sure, as @Effeietsanders says, this is by design. I’m happy to be proven otherwise but I really cannot think of any use case where it really matters that days follow local time − after all, midnight is a somehow arbitrary limit too (uploaders might have uploading sessions spanning 11PM to 1AM). Also, countries may be spanning several time zones too ;-)
If someone can present a very strong use case for having this data follow local time, I’d look into it, but otherwise this is a minefield I’d rather not venture into :-)