$HOROLOG is one of  the "Caché ObjectScript Special Variables".  You can find more details in the link:

http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=...

The documentation about Caché ObjectScript Commands, Functions and Special Variables can be found in the link:

http://docs.intersystems.com/latest/csp/docbook/DocBook.UI.Page.cls?KEY=...