Given a start time in A2 and two duration times in B2 and C2 expressed as number of minutes, the formula for end time would be:
=A2+B2/1440+C2/1440
Use the Number command in the Format menu in ClarisWorks or the Number Format command in the Table menu in ClarisImpact to set the end time cell to display as time rather than a decimal.
Times are expressed internally as a decimal value in ClarisWorks and ClarisImpact, with midnight being equal to 1 and noon to .5. The number of minutes in a day is 1440 which we used here to reduce minutes to a fraction of the day so that we could add it to the start time.