No! Time, thou shalt not boast that I do change:
Shakespeare addresses the ideas of change and growth in one's lifetime by metaphorically standing up against time Father Time. The major theme is that years continue to pass and the narrator is naturally getting older with each passing year, but he does not feel that it is necessary for his character to change accordingly. There are changes in the physical world that may happen within one's lifetime (pyramids), but that is not substantial on a personal level. Even so, we ought to respect what was done before us, however that does not mean we have to revere it and at the same time an individual's pride would persuade one to think of these idea's as one's own, rather than something merely copied from the past (lines 5-8). There is little point in worrying about what has already happened, or for that matter worrying about what is happening now, but one should just live one's life for what it is. Copying down events and comparing written records with mental recollection is pointless because it wastes time in the present to do so, and time is continually moving (lines 9-12). Finally, the narrator resolves that no matter what happens in life (as new events to come are "done" by Time) he will stick to his own constitution and be true to himself regardless of what any consequences may be.
Though this is one common analysis of Sonnet 123, there are numerous takes on the sonnet ranging from the poem's use of time (or lack thereof) as a metaphor for the tyranny of post-modernist working life as well as its potential socio-political themes apparent in the poem's thematic fear of change (conservatism.)
It may also be important to note that this sonnet is one of the few pieces in Shakespeare that references ideas such as time, change, and death without the use of direct biblical or literary allusion.