The hard part for a human is converting the dates you see into standard internet time format (seconds since 1/1/1970), though there is probably a utility program on a website somewhere that can do it for free.
It's all pretty obvious other than that. Here's the first part of my bookmarks.adr file for an example:
Opera Hotlist version 2.0
Options: encoding = utf8, version=3
#URL
ID=47
NAME=Index of /mageia/distrib/2/x86_64/media/tainted/release/
URL=http://mirror.dacentec.com/mageia/distrib/2/x86_64/media/tainted/release/
CREATED=1347506989
VISITED=1347508823
UNIQUEID=46250650FD5311E1A92BFCF391038C72

You see the bookmark starts with #URL. CREATED is the time the bookmark was created (in internet time format), VISITED is the last time you actually visited it ... and there may be a few other lines there if you've set a nickname, description, or have it set to display on the bookmarks bar. If you see an entry that starts with #FOLDER, then it is a folder ... and if you see a hyphen on a line by itself that is the end of the previous folder. (Not that folder structure really matters to you here, just including it if you wonder what all the other stuff in the file is about.

can you please show me by steps on how I would do that? I would really like to reset back the original dates.

It's all pretty obvious other than that. Here's the first part of my bookmarks.adr file for an example:

Opera Hotlist version 2.0
Options: encoding = utf8, version=3

#URL
ID=47
NAME=Index of /mageia/distrib/2/x86_64/media/tainted/release/
URL=http://mirror.dacentec.com/mageia/distrib/2/x86_64/media/tainted/release/
CREATED=1347506989
VISITED=1347508823
UNIQUEID=46250650FD5311E1A92BFCF391038C72

You see the bookmark starts with #URL. CREATED is the time the bookmark was created (in internet time format), VISITED is the last time you actually visited it ... and there may be a few other lines there if you've set a nickname, description, or have it set to display on the bookmarks bar. If you see an entry that starts with #FOLDER, then it is a folder ... and if you see a hyphen on a line by itself that is the end of the previous folder. (Not that folder structure really matters to you here, just including it if you wonder what all the other stuff in the file is about.

Version of Opera? In Opera 12.x and earlier it isn't too hard, as the bookmarks file bookmarks.adr can be edited in any text editor. In Opera 20 you'd need a database editor ...

yes I'm using opera 12, how would I restore the original dates? Thank you for the reply.