Right now it is using the GnomeDateEdit signal 'date_changed', which
unfortunately is emitted every time. If you know a better signal,
please let me know.

Wouldn't it make more sense to check the data when the window is
closed or a 'Save' button is clicked?

That way several details can be changed as a group.

Doing this would remove 'instant apply' which I believe is a better 
way to do it. Also, this would be a coding nightmare.  All the key 
editing operations would have to be consolodated, leaving much more 
room for errors.  Most of the key properties are generated on demand, 
not in glade, which means that to access them, I'd have figure out 
some kind of object to encapsulate everything.
If everything else is satisfactory, I don't think taking 
away 'instant apply' is worth it. libegg, then hopefully gtk, will 
address the incremental 'date_changed' signal in the (hopefully) near 
future. When that has been done, the passphrase will only need to be 
entered once for a date change.

Could the change be applied on the loss of focus instead of the date_changed signal?

Added another dialog to handle this. There were too many problematic issues. Not
only the continual prompting for passwords, but both the date edit control and
the passphrase prompt dialog tried to grab keyboard focus resulting in a real mess.