I figuring stuff like this out because then it makes it so much easier to remember. :)
While it has the same meaning, I don't think it's a valid translation. That would be closer to Min födelsedag är i maj.
So the Swedish sentence "min födelsedag är i maj" doesn't actually translate to "my birthday is in May"?
It sure seems a reasonable version to me. I also can see it referring to one that has completed a year of something, perhaps a job.