Template preprocess function to modify day entry in month view

I'm trying to rewrite Booking Time Slots module to Drupal 7.
In Drupal 6.x the module used 3 tpl files for month, week and day views along with template_preprocess functions to add it's data into calendars.

In Calendar 7.x there are some new tpl files for month including month-row.tpl.php and month-col.tpl.php. The last one as I understand is responsible for single day in month view and renders it mainly with $item['entry'].
Now my problem is I can't find the function responsible for generating $item['entry'] value.
Is this a preprocess function and could it be altered with custom preproces function or is this hardcoded and can't be changed?

Please could someone point me in right direction? Thanks!