On there leaderboard webpage it says:
This promotion runs from July 1, 00:00 to July 31, 23:59 (UTC -8).
Each daily leaderboard runs every day, 00:00 to 23:59 (UTC -8).
I'm not the smartest, fact 
So how does this time translate to CET?
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UTC-8 is 10 hours difference with CEST, so the new days starts at 10:00 CEST.
In winter the difference should be 9 hours with CET.
*disclaimer: i didnt double check and have a severe lack of coffee, so use above info at your own peril 
*
For this sort of thing, I use https://www.timeanddate.com/worldclock/converter.html
On that site, it is possible to create a table of time zone you always look up.
Here is what your query looks like:
https://www.timeanddate.com/worldclock/converter.html?iso=20210701T080000&p1=tz_cest&p2=3922
Lots of other nifty stuff too.
The best way is to check it at the table yourself. When we moved to summer time in Finland, the time stated in website was different than the actual time at the tables.