If the times entered are truly numeric, it should be simple to get their
totals. (To check if the times are numbers rather than strings, you could
try to format them using the 'comma' or 'percent' styles; if they don't
change when you did that, you've got strings; if they do change, then just
'undo' and we can move forward).
If you've constructed the pivot table to show 'sum of time' as the data
field, you probably get back strange decimal numbers as results. You just
need to format these. Right-click on one of the result cells, and select
'field settings.' Click on Number... From the Category list, choose Custom,
then select (or type) h:mm. Click OK a few times a you should be set.
(If the times you have are strings, created a new helper column and use the
timevalue function to convert the strings to times; you'll again probably
see odd decimal numers, which you can correct by applying that same custom
h:mm format. Note that dates and times in Excel are just numbers with
special formats applied.)
