They have their FAQ and your question has been answered there (see quote below). If you want more information on how coinmarketcap works, just go to https://coinmarketcap.com/faq/
Quote
In what time zone is the site based?
Data is collected, recorded, and reported in UTC time unless otherwise specified.
At what time is the 24 hour % change based?
It's based on the current time. It's a rolling 24 hour period.
Data is collected, recorded, and reported in UTC time unless otherwise specified.
At what time is the 24 hour % change based?
It's based on the current time. It's a rolling 24 hour period.
That's also why it is almost completely useless to a trader. You want to the % change against a fixed reference point so you can spot things that are making large moves. Using a rolling 24hr means that the % can also change rapidly due to a move that happened yesterday. All fiat trading systems use a fixed reference point including forex markets that trade 24/5.
