Ohhh... indeed; we can use ajax for both cases depending on configuration. But working on a javascript less solution tat does not lost all input woudl be a plus ;)

Adding an alert before saving would be a good idea; but if this still permit to add an overlapping task this may not be ok for all usages as far as I understand.

The "ajax" alert would replace (or consolidate) the existing. +1 anyways :)

FYI; it is planned to reintegrate the plugin in the core; see https://github.com/glpi-project/glpi/issues/1218