all daily limits erased and now I can't use my system

To solve the situation, easiest way is to delete configuration for particular user.

I'm assuming You didn't alter any other configuration, like "Track inactive sessions" or tracking tty / x11 / wayland, etc.
If You didn't, then just log in via console (press ctrl+alt+f2 and log in, with default config, all console logins are allowed) and erase configuration for particular user, that is sudo rm /var/lib/timekpr/config/timekpr.<your user>.conf
After that just reboot, timekpr will create new fresh configuration and You'll be able to log in again via normal GUI login.
Please be careful with intervals and time limits, do not lock Yourself out (as warning in admin app says)!

Please write back whether You succeeded.

When do I press ctrl+alt+f2 to log in?

I’ve been hitting those keys repeatedly at every stage or Mint startup, but it hasn’t done anything. Mint keeps logging in as usual, counting down, and logging off—etc, ad infinitum.
I’ve been able, after practicing enough times, to navigate to that config file in the file manager, just in time to see that of course, I can’t delete it without the right privileges.
So if you could help me find another way to login I’d be very grateful.

You can press it when Your normal graphical login shows.
It some sometimes appears with little delay, wait some 30 secs, if it doesn't appear right away.

Oh, while I was answering, You already wrote answer.
Do You have autologin enabled?

Yup, it seems like autologin is preventing me from logging into console mode.
If by “when your normal graphical login shows” you mean when the login screen shows, before it logs me in, I’ve done that...
Any other ideas?
Is it possible it’s another key than f2? I’ve tried f1 as well... to no avail.

Yup, it seems like autologin is preventing me from logging into console mode.
If by “when your normal graphical login shows” you mean when the login screen shows, before it logs me in, I’ve done that...
Any other ideas?
Is it possible it’s another key than f2? I’ve tried f1 as well... to no avail.

Yup, it seems like autologin is preventing me from logging into console mode.
If by “when your normal graphical login shows” you mean when the login screen shows, before it logs me in, I’ve done that...
Any other ideas?
Is it possible it’s another key than f2? I’ve tried f1 as well... to no avail.

Well, it's usually like this: f1 or f7 is graphical login, the ones in between are console.
But if You have autologin, that complicates things.
I'll try to search web how to prevent that in Mint.

Thank you.
And btw, I hit Solved by accident. Typing all of this on my tiny phone screen.

Which version of Mint You have?

If You can e.g. have another computer at hand or even mobile phone, try
installing ssh client and login via ssh.
You need to know that computers ip address and hope that ssh server starts by default :)

19.3, Cinnamon (mentioned that in my initial post ;))

Sorry, i don’t know what that means. But I’m assuming it’s a way to control my computer remotely?
In any case I don’t know the machine’s IP; how can I find it out? (Again, I am able to boot into Windows.)

Might there be a way to login via recovery mode that allows me to access the config file?

I'm not familiar with Mint so much, but if it has recovery, that's Your saviour.

If You log in in windows, most likely the ip address in linux is the same.

I booted into recovery mode —> drop to root shell prompt.
What’s the command for deleting the config file, after I’ve navigated to the directory? Not very experienced with command line...

Ok, seems plausible :)
Try this:
cd var/lib/timekpr/config/
ls -la
sudo rm timekpr.<user that looks like yours>.conf
sudo reboot

Saise, got it a little wrong (also tying on mobile :D)
 cd /var/lib/timekpr/config/ please note / before var...

Done. (I found the rm command -- although I didn't do "ls -la"... what does that do? And I didn't need to sudo since I was already in root...)

Damn that was long. But it's good to have my computer back :D

FYI, I think I WAS careful when changing my intervals initially; I certainly didn't delete the daytime intervals. But as I wrote I got an error message when I clicked on "Apply daily limits," so is it possible there's a bug somewhere?

Anyway, thanks for your help, and thanks for this program, it's been awesome to me (until now ;)).

Please send me the log file /var/log/timekpr.log, I'll try to investigate.
If You deleted all of intervals, this might happen.

Glad it's solved, it took some time, but solved :)

Please zip it before sending it might be large...

Emailed you at <email address hidden>!

Admin application now allows to set multiple day configuration at once. I changed logic, hopefully this will not happen again (I could not reproduce, so fix is tentative anyway).

Currently in beta, please check it out when it'll be available in my ppa (should not take more than one hour).

Now, while testing more, I might have a clue how this happened... But now the measures are in place, this should not happen anymore.