Simple Date Subtraction

Simple Date Subtraction

I'm having trouble doing a basic date subtraction in a Zoho CRM custom function. I've got a future date/time that I need to subtract one day from, and then use that new date/time to create a calendar event.
Here is the code:

fDateStartTime=ifnull(asps.get("1st Date/Start Time"),"");
fDateEndTime=ifnull(asps.get("1st Date/End Time"),"");
signWaverStartDateTime=fDateStartTime.subDay(1);
signWaverEndDateTime=fDateEndTime.subDay(1);

This produces an error saying it can't find the subDay function. If I change to the other format (subDay(fDateStartTime,1)), I run into data type mismatches, and all my attempts to employ things like toDate and toString don't work. It's all very confusing.

I'm sure I've done something very basic and incorrect. I'm new to Deluge and programming in this decade :).
Thanks,
Doug