Yes the Greek/EZ crises did have legally defined deadlines because without an agreement by a particular date then the Greeks would have ran out of liquidity, a bit like Democrat/Republican talks doing the same and leading to a government shutdown when they fail - but it would have been a default instead of a shutdown. The talks frequently went to the wire.
May's WA was rejected by our Parliament yes, but then the EU refused to reopen the WA for new negotiations and called the matter closed. May tried to get it amended and failed. In the Tory leadership election Boris pledged it would either need to be reopened or no deal - and everyone insisted that it wouldn't be reopened . . . only for it to be reopened and changed as the clock was running out. Same principle. The EU could have reopened it sooner, they refused until the clock was nearly out.
Neither side were willing or able to work to any particular deadline as each party was waiting for the other to move - and refusing to do so. If the UK had moved then the EU would have banked that and told us to move further. How were we supposed to get the EU to move other than waiting until the clock forced it to do so?