There are several ways how to combine several measures into one measure. You can use either the CASE statement or IIf statement.
For example, you can define a new calculated measure in the Measures dimension with the following formula that uses the IIf statement and the DateCompare function to compare the date in each report row to the date of today:
-- compares the time member in rows to today
DateCompare([Time].CurrentHierarchyMember.StartDate,"today") < 1,
-- if the time member in rows is today or previous days then history
-- if the time member in rows is after today, then custom formula