Problems with using the bootstrap-datepicker in Fitnesse Tests

In my Fitnesse Tests I want to enter dates through datepicker elements. Sometimes it works. But most of the time a different date, unlike the date that was entered, appears. Here is an example:

| ensure | do | type | on | id=field_id | with | |
| ensure | do | type | on | id=field_id | with | 05.05.1997 |
| check  | is | verifyValue | on | id=field_id | [28.05.1997] expected [05.05.1997] |

(To make sure that the field isn't already filled, I pass an empty String first.)

Mostly, the 'day'-statement is different from what was entered. Do you know the reason for this behavior? How can I solve this?

Thanks in advance!