Thanks for correcting my mistakes regarding my first post.
I think I should break down the story to make it clear that "since yesterday" was not wrong to use in one of the above sentences.
A guy went to his office yesterday, even he was sick. He spoke to his manager about his condition, and wanted to take short leave.
The manager excused, saying "The boss is not here, when he comes I will ask him and let you go if he agrees".
Their boss came to the office for a short while, and then left the office for a meeting outside. The guy who was sick worked full-time.
The manager was so busy that he couldn't even remember asking the boss.
Before closing the office, the manager remembered it when they were saying "good bye" to each other.
The manager said "I am sorry, boss came for a short while today, and I was so busy that I forgot that you were sick. I couldn't even remember asking him".
So the poor sick guy went home very tired.
(In the above story only the manager knows that the guy is/was sick, but boss doesn't/didn't).
The next morning the guy wakes up late, and feels that he is still sick and can't go to work. He receives his manager's call after half hour of his waking.
The manager says "Are you ok"?
The guy says "Sir, I am still sick. I am sorry I won't be able to come today".
The manager says "It's ok. Take rest. Have you informed the boss? Boss hasn't come yet when he comes I will let him know, but you also have to text or call him about your condition".
The guy says "No, sir, I haven't informed. I woke up half hour ago. I was washing up. I will text the boss now".
The manager says "ok, Take care, bye"!
The guy then texts his boss "I couldn't come to the office today due to being sick since yesterday".
(In my opinion the guy used "since yesterday", just because he wanted to let his boss know that he was sick, even then he was at work yesterday).
I would like to know the difference between context and sentence.
