How to change the date of rewarded User Points?
Have a look at the User Points Contributed modules as a possible alternative. I.e. its submodule userpoints_retroactive. Excerpt from that project page:


  Award users points for their nodes, comments and votes that they have created so far, before userpoints was enabled on the site.


Comment nr 9 of issue nr 1187616 contains a patch with these enhancements:


Ported the module to Drupal 7.
Added support for retroactive revisions.
Made provisions to store the timestamp of original node and revision.


So instead of trying to change the dates of the User Points transactions, I'd delete the transactions you already have, and try to recreate these transactions using this patch.

PS: there is also Berdir's Api documentation about this submodule ... (maybe that helps somehow).