What's he saying?
"No, Time, thou shalt not boast that I do change: / Thy pyramids built up with newer might"
Time, you won't change me. Worldly buildings and monuments
"To me are nothing novel, nothing strange; / They are but dressings of a former sight."
Aren't special to me, but are merely new disguises on an old structure.
"Our dates are brief, and therefore we admire / What thou dost foist upon us that is old;"
Our lives are short, so we have great respect for old things;
"And rather make them born to our desire / Than think that we before have heard them told."
We see these old things dressed up new, and make them what we want them to be instead of realizing that we have seen them before.
"Thy registers and thee I both defy, / Not wondering at the present nor the past,"
In defiance of Time and its records, I refuse to be in awe of anything of this time or a time past,
"For thy records and what we see doth lie, / Made more or less by thy continual haste."
The recorded events of certain times are made more or less important by time's quick passing.
"This I do vow and this shall ever be; / I will be true despite thy scythe and thee."
I promise to be faithful despite Time's destruction.
Why is he saying it?
Sonnet 123 begins the last three sonnets dedicated to the fair lord, in which the poet returns to the idea that no matter what Time brings, his love is not affected like all other things on earth. The opening line reads like a challenge to time, "No, Time..." no matter what you do, you will not "boast" a victory over my true love. This challenge is reaffirmed in line 9, which begins the third quatrain, "Thy registers and thee I both defy." The use of the word "defy" is pugnacious; the speaker is not going down without a fight.
The idea of dressing up something old and tired to present it as new, put forth in lines 3-4, is reminiscent of Sonnet 59: "If there be nothing new, but that which is / Hath been before, how are our brains beguiled, / Which, labouring for invention, bear amiss / The second burden of a former child!" In both cases, Time is characterized as a trickster who deceives living people by presenting them with old things, disguised as new things. This idea is enforced in line 6, when the speaker references, "What thou dost foist upon us that is old;" the use of the word "foist" suggests a cheater at cards or gambling.
This sonnet is a commentary on people's tendency to be tricked into thinking old things are actually new, and of their own making. In line 7, the word "them" refers to the old things Time is accused of "foist"ing upon people. People, preferring novelty and self-flattery to old, tired things, view these things "born to our desire," or as if they are the viewer's own, newborn creation. The word "born" is a pun on "bourn," which means to border or limit. The pun is that these things are made young in the view of the beholders in order to satisfy the extremes of their desires.
The meaning "pyramids built up with newer might" in line 2 is debated by scholars. It could be that "pyramids" means all lofty constructions in a general way, which will inevitably fall victim to time despite being built with the intent to outlast it. The use of "thy" rather than "the" implies that these structures belong to Time after the people who built it have also fallen victim to Time. However the phrase "built up with newer might" suggests that the "pyramids" might specifically refer to the towers built as part of the pageantry for the coronation of James I in 1603-04. Many poets were praising James I as well, though this poet stays true to his fair lord.
The final couplet is the declaration of the poet's vow to be faithful despite Time and its "scythe." The "scythe" is a tool used to mow grass to make hay, and is a commonly used implement of time; a metaphor for how Time mercilessly mows down everything that is alive. The word "thee" can be understood to refer to Time, separate from its implement, the scythe. However, it could also be a veiled reference to the fair lord himself; the poet remains true despite Time's scythe and despite the youth's unfaithfulness.
