"Does a second have any independent existence, or is it just something that has emerged from arbitrary divisions of years and days?"


A second is a measurement of time, a unit defined by us.

Time (perhaps a tool we use to make sense of what we term change) exists. Unless you’re a
photon moving at c, as I understand they experience no passing of time.

Since we exist in space-time, the faster we move in space the slower we move in time (time dilation).

Time is measured in its own frame of reference, so if you have two observers in different FOR they
will not measure a time interval the same.

Photons travel at c in all FOR no matter how fast we are moving.