Dates and times are stored as whole numbers and decimals respectively (i.e. 6 hours is 0.25 of a day) so to add your expected run time which is specified in minutes, first divide it by 1440 (number of minutes in a day) and then add it to your date dimension.
Hope that helps.
What about DATEADD('minute',[Expected Procedure Length], [Start time])?
If your data is already a date data-type, Andrew's response of
DATEADD('minute',[Expected Procedure Length], [Start time])
will get you there.
You would need [Expected Procedure Length] to be a field in your data, or you could hard code it as:
DATEADD('minute',90, [Start time])
One additional option would be that you could make [Expected Procedure Length] a parameter as an integer value.
That way, instead of it being static, the user could select from a list of choices or input any value you wish to allow.