How to create a Drupal calendar with 2 digits (day date format) instead of 1? Does anybody have experience with adding a 0 before the numbers 1 to 9.
After this my calendar numbers should all have 2 digits: 01,02,03,04,05,06,07,08,09,10,11...

I have added a new PHP date format ("d") and that should give 01 instead of 1 (according to the PHP manual).

But I don't know how to implement this in my calendar.
At first you should add a date format on that page admin/config/regional/date-time/formats/add.

For example this format (F d, Y) gives you that result: September 9, 2015.

Then you can use it anywhere on your site.