In the tick event you should compare the difference between the current date and time and the datetime the user has entered.
There is an example in the book "Mastering Visual Basic .NET" by Evangelous Petroutsos ...
Listing 9.16[
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The trick is not to check for equality (see the comment by @Ralf-Meier) - compare the
difference between the current time and your user-entered time - again, see the example on the link.
Be aware that there are 4 Timer classes in .NET that behave differently - see the
documentation here[
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Finally, terminology counts .. you are saying that you have written a "program" to click a mouse. You have actually written some code to do that. The Timer is not running a program it is running your code that is in a separate function.
If you want to run a specific program at a specific time then you might be looking for something like
Task Scheduler for developers - Win32 apps | Microsoft Docs[
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