Do postdoc positions (joining date) start only in the fall semester or in the spring semester also? I mean, can it start at any time during the year?
Postdocs are often so informal that they can start almost any time.
I had one starting in early August and one starting in February, both not at the start of either semester.
The start of a postdoc is far more informal and fluid than the start of either graduate studies (which are governed by the academic calendar) or many faculty positions with teaching obligations etc. to them.
They essentially start when the PI says they start - or more accurately, a mutually agreeable date between the PI, the postdoc, HR, etc.
For example, my postdoc started in the summer. I am currently looking for postdocs whose starting date is "When can you get here?"
The start of a postdoc is generally dictated by two things:
- The time when its funding starts, and
- The time when people are available to fill it.
While the first can be any time, the second is often aligned with academic cycles, simply because that's when people are most likely to be graduating and wanting to join a new position. That, in turn, does tend to align postdocs to be often aligned with the academic year, but theoretically able to start and stop at any time.
One consideration is that in some cases, postdocs may be expected to teach. (This is typically the case in mathematics in the US, for instance.) If so then the start date would normally be aligned to the start of an academic year, or at the very least the start of a term.