NOTE: Since these labels will change over time, I've archived the profile pages to the Wayback Machine.
It seems to jump from 60 days to 2 months:
- This user is a "Member for 60 days"
- This user is a "Member for 2 months", even though they registered on May 11th 00:07 (UTC), which is slightly less than 2 months ago.
The cut off between two and three months:
- This user from April 13th is a "Member for 2 months"
- This user from April 12th is a "Member for 3 months".
- This user from March 13th is a "Member for 4 months".
This would suggest that your guess "Is 30 days considered a single month?" is right. But a year is 365 days, not 12 months:
- This user, registered July 12th last year, is a "Member for 11 months".