Way better than the default GMail setting, but it would be great if this add-on allowed one to choose the date format.
GMail is extremely limited: if one wants to have dates formatted according to ISO-8601 (YYYY-MM-DD) one has to change language to French Canadian or Lithuanian (besides some oriental languages).
ISO-8601 is the format prevalent in scientific work, and one quickly gets used to it, since it follows the usual little-endian number format which is universally adopted.
Chrome users have better luck:
https://chrome.google.com/webstore/detail/gmail-date-time-formatter/nkmaeogddfijadboccpafnmhkjhidgeb

To illustrate it better, a joke:
If the same logic used for dates was adopted for numbers, "three hundred forty five" would be
- 345 in ISO-8601 way,
- 543 in most others way and
- 435 in US way.