time format past 24 hr

[Theo]

I am working with a spread sheet that reports (imported)
time values in a text string. The calculations performed
on this string return a value in time format (hh:mm) but
if the value is over 24 hrs it returns only the value
between 24 hr increments (12:03 = 12:03, 25:03 = 1:03,
49:03 = 1:03, etc). I want to keep the extra-24 hr format
for time totals that are more than 24 hr.

Thanks
Theo