That ocho días is a week and quince días is two weeks is probably one of the first "quirks" of the language my 8th grade Spanish class was taught.
But now, lo these many years later, I'm still unclear on how to faithfully render the English eight days (not one week) or fifteen days (not two weeks) into proper and unambiguous Spanish.
Tenemos que regresar en ocho días a partir de hoy.
Mi credencial vence en quince más días.
Do these, or either of them, say what I mean?