Yep, days and months. I was able to fix it by running several queries such as:
Update `mybb_users`
set `birthday` = REPLACE(birthday, '01-', '1-');
Update `mybb_users`
set `birthday` = REPLACE(birthday, '02-', '2-');
etc...
that would catch both the day and month portions. I'm used to Oracle and Teradata so I was expecting to be able to use a regular expression to fix them all in on fell swoop, but I soon found out that it seems MySQL doesn't support that...or at least I never got the syntax right.
On a side note, I knew my members weren't going to like losing their avatars, so I devised a few queries to do all the work for me. I copied the contents of the phpbb avatar directory over to the correct location for my mybb install. Then I ran these queries on my phpbb3 db:
//uploaded avatars
SELECT Concat( 'Update mybb_users set avatar = \'./uploads/avatars/abb8d198a5beccd4dc3361807e66127a_', user_avatar, '\'\, avatartype = \'upload\'\, avatardimensions=\'',user_avatar_width,'\|',user_avatar_height, '\' where username = \'',REPLACE(username,'\'','\\\''),'\'\;')
from phpbb_users
where user_avatar_type = 1
//Remote Avatars
SELECT Concat( 'Update mybb_users set avatar = \'', user_avatar, '\'\, avatartype = \'remote\'\, avatardimensions=\'',user_avatar_width,'\|',user_avatar_height, '\' where username = \'',REPLACE(username,'\'','\\\''),'\'\;')
from phpbb_users
where user_avatar_type = 2
Those queries actually created all of the update statements needed to update the mybb database with the users' avatar info. One last thing needed was to remove the weird salt that phpbb3 put into some of the avatar filenames. I never have figured out why this only happened to some of the avatars. Anyway, I ran this query:
//UPDATE mybb avatars that had weird extra SALT concatenated into filename
Update mybb_users
set avatar =
CONCAT(substring(avatar,1, instr(avatar,"abb8d198a5beccd4dc3361807e66127a")-1),'abb8d198a5beccd4dc3361807e66127a_',
substring(avatar, instr(avatar, "_")+1,(instr(avatar, substring_index(avatar, "_", -1)) - 1)- (instr(avatar, "_")+1)),
substring(avatar, instr(avatar, substring_index(avatar, ".", -1))-1,length(avatar)))
where avatar RLIKE'[[:alnum:]]+_[[:alnum:]]{1,5}_[[:alnum:]]+[[=period=]][[:alnum:]]{3,4}';
to remove the salt from the filenames in the mybb db. The salt is board specific I believe, but you can find it by looking at some of the avatar names on the phpbb side.
I post all of this not because I think it needs to be in the merge system. On the contrary, I believe it would be difficult to do this on a global level. But, maybe somebody will see this and find it helpful.