Few weeks ago Hannity had a guy on TV and explained the process. I can’t recall exactly, however November and December come to mind for many states. Evidently there is tons of paperwork required for each state, and can vary from state to state. That’s what I know
“When is the last date for a Democrat to jump in?”
That depends on what the meaning of “is”, is.
The US Constitution does not recognize “parties”, “primaries”, “running for President”, “conventions”, or even “voting for President”, except for the 538 Electors.
So, all of the above happen according to manmade rules with no underlying Constitutional structure, and as such are always subject to change.
Even bright line State laws dealing with these matters are subject to last minute court-ordered changes, as we saw in the NJ Senate race in 2002, when a court allowed what New Jersey law specifically prohibited, i.e., replacement of a candidate and reprinting of ballots within 34 days of an election.
So, to address the underlying issue, the Democrats can nominate anyone they wish without regard to prior events up until the third night of their 2020 convention, and, if that candidate appears to be losing, can replace that candidate up to Election Day, and if it suits them for whatever reason, can instruct their Electors (assuming 270+ Democrat Electors have been chosen) to vote for anyone of the party’s choice.