WarcraftRealms.com

I was wondering if its possible to get the date of when a quest was completed.

Recently, it came to me that a very valuable item from a low level was vendored, so Blizzard should be able to restore the item. Of course Blizzard need a date of approximately when the item was vendored. As my warrior is 60 and the quest item is from level 40 I have absolutely no idea.

I hope you have some wise answers.

There isn't a way to do this without mods. There are some mods which, if installed at that time will keep track of the history of what quests were completed when, and if anyone else was with you to complete the quest. What you can do is check your character history here on Warcraft Realms as it will list the date that you were last seen at lvl 40. You can then give Blizzard this information + or - a few days and they may be able to assist you further.