I am not sure what time zone is used here for their information presentation.
Their information on About this ETH gas data points to Gasnow.org which is non existing website.
The charts above use the “standard” gas price given by gasnow.org. It is recommended that users who are going to transact on Ethereum check gas prices before doing so. The heatmap calculates an average of these standard prices for each 1-hour window using data from the previous two weeks.
There is only Gasnow.io that has no information about used time zone too.
That site has some more information than Milkroad.com and even has an extension I will not install it, only visit the website to get information I need.