title: How can a program run at specific time?
body: <p>(Sorry,I do a little change in my answer)</p>
  
  <p>I want to know can I write a program that runs at a specific time?Is there any way to run a program automatically? And another question:Can I write a program that runs whenever another program is executed?</p>
  
  <p>Note that it does not matter what the OS is,and I want a way to be in program code,that is I write a line of code that do the operation,because I cannot prompt different users to change their OS options to do the operation.</p>