Create a Demo Date and Time based on Today() and DateTime.LocalNow()

OK, seems to be some confusion here.

I already have a Calender Time Intelligence table in my platform ... so could use the date on a slider, but that's not what I am trying to achieve ... I'm using a reference to real date and time (i.e. Today() and LocalNow()) and this is embedded across multiple formulae.

I need to set a FIXED DATE and a FIXED DATETIME (System Demonstration Day) in a variable as though I was showing the system on that day from 18 months ago. I don't want or need a slider that will cause chaos and confusion to end users.  It needs to be invisible to users and set by me once for a live system ... i.e. set to Today() and LocalNow() and/or alternatively I can set a fixed date and time instead for a demo system.

These variables are used in about 200 formulae in the system ... so I can't be changing each formula for each implementation.

I thought this would be simple code - but seems to be something ridiculously complex to do.

Surely someone has done this before ... ?