The word year when pronounced starts with a phonetic sound of e which is a vowel sound making it eligible for being preceded by an. Yet, we tend to write a year. Why?
I reject your premise that the word year starts with a phonetic sound of e. Here's how it is pronounced according to the various dictionaries:
- Wiktionary: (RP) IPA: /jiə/, /jɜː/, SAMPA: /ji@/, /j3:/; (US) enPR: yîr, IPA: /jiɹ/, SAMPA: /jir/
- Merriam-Webster: \ˈyir\
- American Heritage Dictionary of the English Language: /yîr/
- Collins English Dictionary: /jɪə/
Words that start with the /j/ sound are preceded by an a, not by an an. Compare: a user, a utility, a yak.
For the word 'year' to be preceded by 'an' it must sound like it's beginning with a vowel. The reason why it is a tad tricky is because of the difference in the way people pronounce it.
Some people pronounce the word 'year' as 'ear' with 'y' silent, thereby wanting 'an' to precede and feeling discomfort with 'a'. e.g. 'It's been an _ear since we talked'.
Whereas others pronounce it with a 'ye' sound in the beginning. In this case one finds that using 'a' suddenly sounds more comfortable. e.g. 'It's been a 'ye'ar since we talked'.