I'm in Pacific time. I've noticed what looks like some sort of timezone mismatch between the 'heat map' and the actual list of activities, on the user activity page. Couple of examples: for 2018-02-18, the heat map shows a fairly dark green square with the mouseover text "18 items on Sunday February 18, 2018", but if I click on it, I see "No activity recorded on that day". The 18 items seem to appear on the list for 2018-02-19.

Conversely, for 2018-02-24, the heatmap shows a grey dot and mouseover text that has no item count, but when I click on it, it shows 12 activity items for that day.

I suspect one thing is adjusted for local time, but the other is not, and is counting in UTC. Or similar.