The language of this sonnet is a bit tricky.
The poet begins by saying that he does not "pluck" his decisions "from the stars." He does not use astrology to "tell of good or evil luck, / Of plagues, of dearths [famines], or season's quality."
Still, the poet thinks that he does have some knowledge of "astronomy," which we would call astrology. He can make predictions by looking into the eyes of his beloved, which he compares to "constant stars."
He sees in his beloved's eyes "truth and beauty," but he realizes that these will only last if "from thyself to store thou wouldst convert." This means that her beauty will only last if she will change (convert) from concentrating on herself, and instead will give herself "to store," which means to producing children.
It is only by having children that the beloved's beauty can be preserved for eternity. If she does not reproduce, the poet prognosticates (predicts):
Thy end is truth's and beauty's doom and date.
When the beloved "ends," or dies, that will be the doom of beauty.
This idea that beauty will fade and disappear, and that only by producing children can beauty become eternal, is found in many places in Shakespeare's sonnets.