The phrase 'I was off today' is correct and usable in written English. You can use it when you need to explain why you were unavailable or out of work during a certain day. Example sentence: "I was off today, so I couldn't finish the project.".
Actually, I was off yesterday, and shopping with my husband, and I saw a man standing in the street, literally counting his pennies before he went into a shop.
"I'm off today, that's another reason why it may not seem so long".
But "it's free, and I'm off today, and I don't mind looking after me".
I'm off next week, back the following.
I'm off next week (half-term).
I'm off tomorrow, possibly lying in a ditch somewhere.
"Can we expect you to be covering the Saudi Arabia-Yemen friendly tomorrow to keep your Lampardian streak alive?" No, I'm off tomorrow, perhaps by popular demand.
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