Interesting: The Arabic word, سلام /salaam/ also means "peace" (in addition to "haven", "well-being"). Arabic is Afro-Asiatic, and Swahili is Niger–Congo.
Swahili has many Arabic loanwords given the history of trade in East Africa.
Can a fluent speaker please tell us about this sentence and when it would be used. The correction I was given was "Today is safe" but I don't understand the context. Is today safe from attack from hostile forces? For "Today is peaceful" is that because all of the kids are away on holiday? Is the real meaning and usage "Today, everything is fine."
And me - but I think earlier lessons used "today" as the translation of "leo" - as in How are you today: Habari ya leo (IIRC!!)
I wrote The day is peaceful, I think this should also be accepted as a translation.