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The work presented here is related to the area of optimal preventive maintenance (PM) scheduling in semiconductor manufacturing systems. Specifically, we develop models and algorithms to convert PM task schedules based on the count of wafers processed into equivalent calendar PM schedules. These equivalent calendar-based tasks can then be used within an optimization algorithm for PM scheduling. We propose a discrete mathematical model and an algorithm to convert from wafer-count targets into equivalent calendar dates. An extension of this model and algorithm is presented to perform conversions of other types of PM's, such as those based on processing time or energy spent, into equivalent calendar-based PM's.