Here is a acro that will do that. No scripting required since Keyboard Maestro already has a bunch of date functions.
If it will always be at least 2 days ago, you can remove that second last action (the If Then Else action), but I never like to say "1 days ago", the extranious s always bothers me.
Days Ago.kmmacros (4.0 KB)
The ROUND in the calculation should not be needed since the dates are always GMT there should be no daylight savings issues or the like, but I left it in anyway.