Past, Present - and Future?

While reading a little from Dom Edmund Martene's De antiquis Ecclesiæ ritibus (Antwerp, 1763), I came across the following rather startling prayer (p. 216), part of the response to the Orate fratres, in a Mass Ordo he quotes as deriving from the monastery of St Gregory in the Gregorian Valley (?) in the Diocese of Basel (if I translate it aright):

Misereatur tui omnipotens Deus, et dimittat tibi omnia peccata tua præterita, præsentia et futura.

(Almighty God have mercy upon thee, and forgive thee all thy sins past, present and future.)

While it makes sense to pray that a person's past and present sins be forgivenen, it seems mighty presumptuous to pray that their sins of the future be forgiven!  How can what has not come to pass be already forgiven?