I have 2 dropdownlists in the Footer of a datagrid:

ddlStartTime and ddlEndTime. Both contain values like "1:00 AM", "1:15 AM",
"1:30 AM", etc.

There is also a label named lblTimeDiff.

I need to dynamically popolate lblTimeDiff.Text (during
onselectedindexchanged of ddlEndTime) with the difference in minutes between
ddlStart and ddlEnd in the Footer. I'm sure javascript will come into play
here but I have absolutely no idea how to define the eventhandler and
scripting. I can do this in tsql using datediff(mi, start_time, end_time),
just can't seem to dynamically put the return value in the label without a
postback to the server. Any ideas? Thank you very much.