I deleted it and resposted it in the proper place in this same thread - I accidentally responded to the wrong person.

Edit: omg, hahaha

They have good cause to believe you’re a Signal user, so they get a judge to authorize a subpoena based on your phone number, and Signal complies - and, yes, all they’re doing is confirming to the FBI that you have an account with them.

Literally all they need to do is have their own phone with Signal installed, and then create an address book listing with the Suspects phone number on it.
Next time Signal syncs, it will pop up “Suspect is on Signal!”.
Subpoena-ing the Signal Foundation is not required.

But it proves nothing, and if you are in a jurisdiction where the police can demand your phone or you are stupid enough to hand it over when they can’t, that’s not on Signal.

deleted by creator

I can’t think of a standalone gui app that does this (and a simple google search didn’t find one).
If you have a gui desktop (gnome,kde,xfce,lxqt,enlightenment,budgie…) it will have a built in function in it’s settings to do this, or leverage one of the parent ones (ie budgie is based on gnome, lxqt on kde).
If your custom environment is pared down to the point where you don’t have an equivalent to gnome-system-tools and don’t want to install it, you might have to just use date at the command line.

Honestly sounds like a job for a Raspberry Pi Zero.
Small battery pack and a USB key, a bit of software setup, off you go.

Install Raspbian: https://www.raspberrypi.com/documentation/computers/getting-started.html#raspberry-pi-imager

Set it up as a hotspot:
https://www.raspberrypi.com/documentation/computers/configuration.html#enable-hotspot

Configure Samba share:
https://www.raspberrypi.com/documentation/computers/remote-access.html#samba