More than once crontab guru have ensured that I didn't schedule a job wrong, but what to do when you're employer decides that it's all systemd timers from now on?
I haven't found any good websites for helping me validate my timers, but maybe that's they don't have to exist.
Systemd actually ships with a tool that pretty good for double checking your work: systemd-analyze.
Just enter systemd-analyze calendar "0/1:00:00" in your terminal and systemd will tell you not only if your timer is valid, it will also let you know when the next run would be. Very handy.