In "Sonnet 106" Shakespeare uses an extended metaphor. A metaphor is a figure of speech in which a word or a phrase is applied to something to which it is not literally applicable. So here, the speaker looks back over the whole of recorded history ("the chronicle of wasted time") and reads the numerous descriptions of beautiful people and the beautiful poems written about them. And in those descriptions of lovely hands, feet, lips, and foreheads, he realizes that writers of yesteryear were attempting to describe the same kind of beauty that the speaker sees in the object of his love.
In that sense, the speaker compares previous descriptions of beauty to an almanac, a book that claims to predict what will happen in the future. All the descriptions of beauty made by poets and other writers in years gone can be interpreted as prophecies of the beauty of the speaker's beloved. And though those of us now living can witness the beloved's beauty at first hand, it is beyond our ability to add to any of the descriptions, the prophecies of days gone by:
For we which now behold these present days,
Have eyes to wonder, but lack tongues to praise.