Dec = dat2dec(Dat)
Dec = dat2dec(Dat,Pos)
Dat [ numeric ] - IRIS serial date number.
Pos [ 'start' | 'centre' | 'end' ] - Point within the period that will represent the date; if omitted, Pos is set to 'start'.
Dec [ numeric ] - Decimal grid representing the input dates, computed as Year + (Per-1)/Freq.