Time unit multiplier for the relative first observation date offset.

If the first observation offset is specified, the observation period will start the specified interval prior to each calculation period - i.e. if the first observation offset is 4 months and the lag duration is 3 months, observations will be taken in months 4, 3 and 2 (but not 1) prior to each calculation period. If no first observation offset is specified, the observation period will end immediately preceding each calculation period.