Edit task time

Hi all,
I try to edit the date (beginning or end) of a task, and there is no way
to change it: when I click on the date, I get the calendar but I cannot
make any modification in it.
The only thing I can do is go to the gantt chart and there slide the end
of the task's rectangle to change the end date. But I can't slide it to
change the beginning date (why ????)

Note that I'm trying to change the date of subtasks!
I'm using mrproject 0.91 on a debian testing.

Thanks for your help,
Laurent