This may be related to: #17080
When selecting the period "Month" for a website that has its time zone set for example as United States - New York and selecting the first day of the month, Matomo is showing the previous months data.
Screenshot comparison between the same month selected but using the 1st vs the 2nd:
URL parameters: index.php?module=CoreHome&action=index&idSite=1&period=day&date=yesterday#?idSite=1&period=month&date=2020-12-02&segment=&category=General_Visitors&subcategory=General_Overview
This causes the comparison feature to show the incorrect month data (I.e. showing October data for November when comparing December with November)
Manually changing the date in the URL shows the correct comparison data
This same behaviour occurs when using the "Week" period and may also be occurring in the "Day" time period as well (But it may be quite difficult to confirm that one)
The Matomo server is located in a different timezone (EU) to the website timezone (US - NY).
it be great to test this and the other issue once the next Matomo update has been released and deployed. It might be fixed already.
It looks like this one is fixed for the websites that had the error. When comparing the same as in the examples above the reports are showing the correct numbers even when selecting the 1st day of the month (As expected)