They’re both really really close to perfect, and any native speaker would understand exactly what you’re saying with no problem, but I’d say them like this:
“I won’t be at my computer much today, I have to be somewhere.”
and
“It’s incredible how time flies! It will be Christmas soon!”
Yeah honestly this has been one of my takeaways too, after hearing about these awful things a while back.
You never know what’s going on in the life of a stranger on the other end of a phone call, so respect and politeness is the best we can offer.