There are certain positions and awards that are time-sensitive based on the date of Ph.D. or the start date of the first tenure-track position. In the former case (date of Ph.D.), doctoral students who are considering graduating in December might want to consider whether it is better to have an earlier date (December's year) or a later date (January's year) on the Ph.D.
When I was finishing my Ph.D., I was really annoyed with one low-energy committee member who dragged his feet about reading the thesis and signing the forms. I could have been done in December, but because of this slow professor, the official date on my Ph.D. was the next year.
At the time, I was disgruntled, though mostly for irrational and personal reasons; e.g. I had set myself the goal of getting my Ph.D. in a particular year and at a particular age, but, since my birthday is in late December and this committee member was so slow, I missed my random personal goal and it looks like my degree took me one year longer than it really did.
In the long run, the slacker professor did me a huge favor. Years later I got an award based on years-from-PhD that I would have been ineligible for if I'd had the earlier date on my Ph.D. For that reason alone, the later date was well worth it.
A December date might be important in some cases -- e.g. you need a particular degree by a particular date for employment reasons -- but if you have no such requirement and if the 'extra' year doesn't make you seem like your graduate program took unusually long (e.g. forever), you might want to consider delaying the defense or the paperwork to get the next year's date on your degree.
Although you seemingly 'lose' a year in terms of how long you apparently spent in graduate school, you ultimately gain that year back in terms of the official start date of the rest of your academic career.
8 months ago