In reality, the x-axis keeps some “double” (“unsigned int”) values. See TAxis::SetTimeFormat and try e.g.:
TDatime(2019,12,1,16,32,54).Convert() // Sun Dec 1 16:32:54 UTC 2019
TDatime().Convert(kTRUE) // "now" in UTC
See also the TDatime and the TTimeStamp classes references and: