R. Yirmiyah says: Just as uncleanliness constrains (the offering of the Pesach (Passover) [viz. (Numbers 9:10)] and (the advent of) spring constrains, then just as the (constraint in) the first is not less than thirty days, so, (that in) the second must be no less than thirty days. But perhaps (an intercalation is to be made backwards] even if spring arrives prematurely? It is, therefore, written (Exodus 13:10) "and you shall keep this statute (of Pesach) in its time"—You add to its time, but you do not subtract from it.