Improve -Sb of plot for Date/Time unit

[ZMAlt]

Hi, everyone,

A guy from China community found the unit for Date/Time in -Sb of plot module is not work. I don't think it's a bug. So, a feature request is here.

# work
echo 5 5 | gmt plot -JX5c/4c -R1/10/1/6 -Baf -BWS -Sb1q -W0.1 -pdf test

# not work
echo 2023-07-07T 5 | gmt plot -JX5c/4c -R2023-1-1T/2024-1-1T/1/6 -Bpxa3of1o -Bsxa1yf -Bya -BWS -Sb0.1q -W0.1 -pdf test

Are you willing to help implement and maintain this feature? Maybe

[joa-quim]

Looks bugish to me.