I think there is much sadness in these opening lines, they reflect time seen in the waves and time unseen in the minutes, they are comparisons of time passing.
Lines 3 & 4
Each changing place with that which goes before,
In sequent toil all forwards do contend.
I think these lines show us the sequence with which time passes and waves break against the shore, always following on one after the other in a perpetual sequence, unbroken, never stopping always going forward, moving on.
Lines 5 6 & 7
Nativity, once in the main of light,
Crawls to maturity, wherewith being crowned,
Crooked eclipses 'gainst his glory fight,
I think these lines in the poem represent birth and maturity and ultimately the end, possibly his death.
Nativity represents the birth of a child, the birth of a wave and the birth of a minute, the very birth of time itself. The birth of anything is associated with light and beauty. 'Crawls to maturity', again the passing of time, of moving forward, moving towards the end 'wherewith being crowned' the ultimate end. Eclipsed, surpassed as one minute surpasses the next minute.
Line 8
And time that gave doth now his gift confound.
The gift of time can be narrowed down to the gift of each and every minute. That gift of each minute is defeated and overthrown by the next minute, the next gift.
Lines 9 10 11 & 12
Time doth transfix the flourish set on youth,
And delves the parallels in beauty's brow,
Feeds on the rarities of nature's truth,
And nothing stands but for his scythe to mow.
These lines suggest to me that the poet see's beauty and youth as one, a rare moment in life when everything is perfect. His use of the words youth, beauty and nature suggest this to me.
Again the poet tells us that this is time passing, feeding on a rare moment that was here but now is past 'And nothing stands but for his scythe to mow.' reminds us that the time will pass and the grim reaper 'Death' will claim the minute of truth and the time of youth and ultimately the poets life.
Lines 13 & 14
And yet to times in hope my verse shall stand,
Praising thy worth, despite his cruel hand.
I think that these last two lines of the poem suggest that the poet wants to live forever and cheat death and the passage of time. He values time as a gift but he knows that no one lives forever, but never the less he searches for immortality for himself if not in life then in the written word.
I like this poem very much. But I think it is a very sad poem dwelling on time passing and death rushing forward to meet the poet. He does not place any value on the use of each passing minute but only on the fact that minute has gone and will not come again.