Authors are notoriously dilatory. They seem to live by months of Sundays, rather than the typical gregorian calendar month. Although it is hard to get your writing written, with dogged determination, a will of iron, and a hard deadline, it's a little easier. If you're going to make a commitment, keep it. It will earn you long term respect.
And if you realize you can't keep it (Aunt Bertha passes away, your computer falls into the Adriatic Sea, a piano falls on your head and erases the rest of your plot-line), tell whoever you're writing for as soon as it happens. Not three weeks after it was due.
Don't be a 'month of Sundays writer.' Be an, 'on time, good condition, just like I promised' writer. It'll get you a lot farther in the long run.