...Not on that day, but close to it. My own reasons for not celebrating are a bit different.

[The reason is, of course, Alderaan. With Alderaan gone, her birthday only reminds her of what she's missing and of the things she should have had but now never could and never would. So rather than celebrate it, she ignores it.]

But, by my estimation, my birthday was months ago according to the calendar here.

[Or more accurately, counting based on when she arrived, or when Luke arrived. They figured it out together.]