Underestimating the amount of time I need to complete a task

Hi, Brooke! I understand that I should give myself a certain amount of time (at a planned time) to do something and accomplish it in this amount of time – and I know that the more I practice this, the more accurate my estimations of time will get! Meanwhile, when I do underestimate the amount of focused, diligent (no distractions!) time that something will take, what do I do? I have a really hard time leaving something unfinished, but is it better to leave it unfinished and put it back on the calendar at a later time? Thanks in advance for sharing your wisdom!