“reckoning Time, whose million'd accidents”
'Reckoning' here means taking full account of, measuring honestly — as in a financial reckoning or settlement. 'Million'd accidents' means the countless, unforeseeable events that time produces; 'million'd' is a coinage stretching the number into something vast and unmanageable. Together the phrase says: once you honestly factor in all the random upheavals time causes — eroding vows, reversing kings' decrees, darkening beauty, blunting strong intentions — you see why no love-declaration can be final.
“Tan sacred beauty, blunt the sharp'st intents”
'Tan' means to discolor or darken — as sunlight and weather tan leather or human skin. Time tans 'sacred beauty' in the way weather ruins fine material: what is precious and set apart ('sacred') is nonetheless worn down by exposure. 'Blunt' means to dull the edge of something sharp; 'intents' are purposes or intentions. The line pairs two verbs of steady, slow damage: tanning does not destroy outright but discolors and toughens; blunting does not shatter but gradually robs a blade of its edge.
“When I was certain o'er incertainty, Crowning the present, doubting of the rest”
'Incertainty' is an early modern spelling of uncertainty — not knowing what will come. 'Certain o'er incertainty' is an oxymoron: the speaker was confident precisely because he acknowledged he could not be confident. 'Crowning the present' means treating the present moment as supreme, like a coronation, while 'doubting of the rest' means staying unsure about everything beyond it. The logic: he claimed 'now I love you best' only because he knew the future was unknowable, so 'best' referred only to the present.