There are a couple of thinjgs you can do.  The date is stored in ow_blogs_post as a timestamp which means that it has this format  1369788358 .  The higher the timestamp the greater the date.

1. you can look at a timestamp of one of your later blogs in the table and just copy and paste that timestamp into your new timestamp field and decrement it by one.   So lets say that you find a blog date on your site and you want to make it earlier than that one.  Just find that row on your table for the one you want to copy and copy the timestamp lets say for example it is  1369788358, then you just paste that and decrement it a couple like so  1369788348  (see i changed the last two to 48).  This will make the date earlier.

2. (this one is probably easier for you)  Just go here and enter the date and copy and paste the timestamp into the timestamp field of the one you want to change.

http://www.unixtimestamp.com/index.php

enter the date and time (time is optional) and then you can copy and paste the timestamp

if you click on the field in phpMyAdmin you can edit it inline that way