Calendar Changes To Different Time Format

I have a DateTime parameter called End Date set up with a default value eg =DateAdd("s",-1,Today).  When I preview, the default value appears as 19/09/2006 23:59:59, which is fine. 

However, when the report is deployed, it is displayed as 19/09/2006 11:59:59 PM.  This would also be fine, except when the user changes the date using the calendar, it converts the time back to 24 hour time - eg 19/09/2006 23:59:59.  Then when the report is run, it converts the time format back to AM/PM again. 

Also, I have a Start Date Parameter, with a default value of 12:00AM today.  I would like this to be displayed as 19/09/2006 12:00AM in the Start Date parameter text box, however reporting services will not display the 12:00AM part.  Using FormatDateTime in the Default value will return a string and cause an error.  I don't want to change the parameter to a string and lose the date picker.

Any ideas,

Thanks

PS Getting the Date/Time settings changed on the reporting server would be extremely difficult!