Unix timestamp (actual reference with easy math) + timezone give you absolute time to work with, such that you can work out the correct presentation for the timezone even if the timezone has undergone multiple revisions in the intervening years.
There's still a bunch of problems (timezone of event origin? Event reporting origin? Server? Client? Client + presentational overlay?) and confusing things to work through (including relativistic problems) but the consistency of the reference point is valuable. Nobody is going to confuse what timezone you are using as your point of reference when you use Unix epoch. Just use a 64-bit number to hold it.
There's still a bunch of problems (timezone of event origin? Event reporting origin? Server? Client? Client + presentational overlay?) and confusing things to work through (including relativistic problems) but the consistency of the reference point is valuable. Nobody is going to confuse what timezone you are using as your point of reference when you use Unix epoch. Just use a 64-bit number to hold it.