Valid:
<year>1999</year>
<year>99</year>
<year>1999 C.E.</year>
<year>
The year of our Lord one thousand, nine hundred, and ninety-nine
</year>
<year>
Delicious, delicious. Oh how boring.
</year>Invalid:
<year>
<month>January</month>
<month>February</month>
<month>March</month>
<month>April</month>
<month>May</month>
<month>June</month>
<month>July</month>
<month>August</month>
<month>September</month>
<month>October</month>
<month>November</month>
<month>December</month>
</year>