Currently, SQL entries are being stored as text in the format:
YYYY-MM-DD HH:MM:SS

Theoretically it would be better if times were stored as unix timestamps, then converted for display in all related utilities. It would allow for better localization formatting for other things, and would also make time diff calculation far easier.

Whilst it would be better, converting a database of 20 million entries would take absolutely ages

You are correct, it would. (BTW, how are you even determine record ages with the current timestamp scheme. o_o)

That is a standard timestamp accepted by many languages. MySQL itself uses it. Most languages can just read the format off the same as reading UNIX time.

Ah, I've never used it. I will always prefer UNIX timestamps. :)

There isn't much of a difference except as you said - localization is easier. In a query you can still do SELECT * FROM database WHERE date < '2011-08-20 11:28:00'