[SOLVED] How to create a Timer to switch off a light?

[lipp_markus]

First of all, please use code fences:

How to use code fences

We all have to type this content over and over and over. Here is a posting we can all point to when the topic comes up. Why use code fences? Because rules, sitemap, logs, and configs are all but unreadable when code fences are not used. When to use code fences? Whenever one is posting any sort of code, config, log, and any text where line wrapping, indentation, and using an equal spaced font makes the text more legible. How to use code fences? Use 3 backticks (```)... Or use <code>... …

You are not saying whether you see any errors in your log file:
https://www.openhab.org/docs/tutorial/logs.html

Do you use VSC?

Editors

Does it show you any errors?

Please check that you are using straight " (in your post they were the curvy ones”, but that may be a copy paste problem.

What is very helpful in troubleshooting is adding a logInfo statement immediately after the then line of your rule, that will log when the rule triggered. Doing so will allow you to determine whether the rule triggered at all:

rule "switch on"
when 
     Time cron "51 0 22 ? * * *"
then
     logInfo("Switch on","My switch on rule triggered")
     SmallAquariumLight.sendCommand(OFF)
end

Please note that the syntax <item_name>.sendCommand(command) is much preferred over the syntax you have used. read here if you are interested in the why:

Rules

And almost forgot: can you switch your light on and off through a user interface? If not, check your Item or Thing definition.