Historical Linguistics and Language Families 8, Exercise 3: mat and mate

The Great Vowel Shift is dated c. 1400-1700 in the page's timeline, "mainly
during the 15th century" in the text, and c. 1500 in the phonology section.
It "affected the qualities of most long vowels".

It occurred after the spelling system was fixed. So the spelling kept the
older relationship between a short vowel and its long partner (the silent -e
marking the long vowel in mate), while the pronunciation of the long vowel
changed. The result is that mat and mate now differ in vowel quality, not just
length.

The exact quality changes are not given in the part of the page I read, so
this solution does not state them.