Only agree to realistic deadlines

If you know you can’t deliver by the end of June, say so up front even if it means annoying your boss or the customer.

Postponing the annoyance to June 30th may seem like a good option now, but it guarantees you a load of stress trying to deliver the impossible, not to mention really pissing them off when the deadline passes with nothing delivered and no time for them to look at other options.