Slug lines should be all but invisible. They are not there to give the reader crucial information, especially in a spec script. If you do something to make them stand out, you probably have lost your reader.
That being said, anything that helps tell an awesome story is great. Good writers use the slug line as a tool in shaping the reader's expectations and emotions. If your story is awesome, creative use of screenplay convention will not kill you.
But the idea is to serve the story, not be technically correct. Why does it matter exactly how much time has passed between scenes? Is it essential to your story? If so, put it in the action prose. Many people who read lots of scripts don't read the slug lines. In fact, many Hollywood types only read the dialogue, so if it is that essential, have someone say it. Without making it too expository.
Think about how would you say it to someone if you were telling them the story in a bar? And put that in your script.