you have 2 choices
1) submit the form when the user selects a time. query the
database based on the time selected to get the available pieces of
equipment, re-load the form with the query data in the 2nd drop
down. (i don't endorse this choice)
2) use AJAX. it's very much the same as choice #1, except the
form does not submit when the time dropdown is changed. you use
AJAX to go back to the server, query the database to determine
which pieces of equipment are available for the selected time, then
use JS to populate the 2nd drop down. this all occurs behind the
scenes to the user.
I've been using the JSMX AJAX library recently for a project
and liking it a lot. it's fairly lightweight (just 2 .js files),
but you do need to be comfortable with JS in order to do something
with the data that's returned from the AJAX call.
Spry is Adobe's AJAX library, which I think makes it a bit
easier to "plug in" the data from the AJAX call after it's been
returned.
JSMX can be found at
http://www.lalabird.com
Spry can be found at
http://labs.adobe.com/technologies/spry/
Additional info on Spry can be found at:
http://carehart.org/spry/spry_resources.cfm
and
http://ray.camdenfamily.com/index.cfm/AJAX