DM Measure expressions ISNULL, Current or Future Periods

Summary:
1) How can I write a measure expression in Demand Plan that will check if the value of another measure is blank or null to make it equal to null. If the other measure has a value then calculate an equation?

New Measure A = IF(Measure B = NULL or BLANK ) THEN NULL ELSE (Measure B + Measure C)

2) How can I write an expression that will check if it is the current period or greater then leave the measure null. If not then calculate an equation?

New Measure A = IF(current period or future periods) THEN NULL ELSE (Measure B + Measure C)