The Democratic National Convention is scheduled to convene on August 19th.
The presidential election this year is November 5th. That's 79 Calendar days between the Democratic Convention and the Election.

All states have a deadline to certify a presidential candidate on a ballot before the general election. There are eight states which may become a problem for Biden (or whoever else is chosen) unless Democrats change their convention date.

In Ohio, their deadline is 90 days before the general election. "Major political parties must certify to the secretary of state on or before the 90th day before the election names of the candidates for president and vice-president"

In Alabama, the deadline is 82 days before the general election. "A political party must certify to the secretary of state no later than the 82nd day next preceding the day fixed for the election the names of the candidates for president and vice-president and the names of the presidential electors."

In Arizona, the deadline is 90 days before the general election. "Each recognized political party must file with the secretary of state no less than 90 days before the primary election the names of the party’s nominees for presidential electors."

In Georgia, "Political bodies that nominate presidential electors by convention must hold the convention at least 150 days prior to the general election, or, in years following the release of a decennial census, 120 days before the election."

In Iowa, "Political parties must certify to the secretary of state no later than the 81st day before the election the names of the candidate for president and vice-resident and the names of the presidential electors. If the national nominating convention of a political party adjourns later than 89 days before the general election the names of the candidates for president and vice-president must be certified within 5 days after adjournment."

In Oklahoma, "Each recognized political party must certify to the state board of elections no fewer than 90 days from the date of the general election the names of the nominees for presidential electors."

In South Dakota, "Each political party must certify to the secretary of state the nominees for presidential electors within three days of the state convention, and no later than the 2nd Tuesday in August."

In Washington, "Each major political party must certify to the secretary of state at least 75 days before the election the names of the party’s nominees for presidential electors and the names of the candidates for president and vice-president."

The Republican Convention is Mon, Jul 15, 2024 – Thu, Jul 18, 2024, which is well in advance (111 days before election) of these deadlines to get names on the ballots.