With all of the faults in the Backlog data set, the definition of End of Month seems to be the most illogical. If the end of the month of September is 11:59:59pm on September 30, then the Backlog dataset should show the backlog as of that day and time. But, according to a Zendesk Technical Support Engineer and the Explore team, I was told:
"1. The backlog data is collected at each sync but is recorded per date not per hour. So, for the Pro accounts the last recorded value is the one recorded at the last sync. The same for the Lite accounts but for them it is the only sync per date.
2. It should be the last day of the period. So, End of month is the last date of the month
So, long story short, as of right now the "End of Month" metrics/queries are expected to start showing data with the first datasync of the last day of the period. In this case, it explains why data started showing up on September 30th before the day had actually ended. "
So the End of the Month of September is actually the first sync on the last day. That does not make logical sense. I should not see End-of-Month data in reports that use the Backlog dataset until the month is over--September data should not show until after 11:59:59pm on September 30.