It use to be valid only "sePtimebre" until a few years ago. I can find the date, may be 2000+something. Now "setiembre" is also valid. It looks like it's popular in Peru, but sometimes it's used here too. (When I say it, the "P" is weak, probably closer to "seBtiembre", but it's there.)

The tradition is Spanish is to keep the orthography synchronized with the oral form, so if some variant gets too popular it's included in the RAE dictionary and later if the old form is unused it's deprecated.

It's very different from English, where the oral and written form of a word may be almost disjoint.