"Mi cumpleaños es en catorce días"
Translation:My birthday is in fourteen days
Or as they say in the UK - "My birthday is in a fortnight". Or as my grandchildren say "Only thirteen sleeps till it's my birthday!"
Why can't you use estar? "Mi cumpleaños está en catorce días"...because it's a time thing. Just wondering...
Since date and time are always 'ser' it's a safe bet that anything referring to a year, month, week, day, hour, minute will be 'ser'. I do not know that for a fact meaning sometimes there are exceptions.
Owls have very specialized eyes. They can only read letters, not numbers. This is why the owl collects so many hearts - he can't read the number on the dial of the heart-collecting machine. So he doesn't realize he has more than enough hearts, and could actually be a little easier on his long-suffering students.
I just used 14, and I have used numerals rather than spelled out words in several of these, and they are being accepted now.
"within" is used to express a range. You could say "His birthday is within fourteen days", but this means that it is on an unspecified day over during the next two weeks. "His birthday is in fourteen days" gives the actual day which is his birthday.