I'm assuming you have a field that accepts a datetime value as well as your
checkbox.  To insert the current time into your datetime field when someone
clicks the checkbox, right click on the check box and go to the Check Box
Properties.  Then go to Rules... and Add a new rule.  Add a new action to the
rule - and from the drop down of actions, select "Set a field's value".  
Select your field and then for the value click on the icon to insert a
function.  Select the "now" function (you might have to click on the Date and
Time category).

Then go back to your time field and select the properties and click on
Format...  Under the "Display the date like this:" list, select (Do not
display date) and then also select your time format from the "Display the
time like this:" list.

This has been pretty detailed, sorry if you already knew most of those
steps, but I thought I would include as much as possible.

HTH
SChapman