HI @aibd,
this is pretty interesting, since when I tried (with an old version of /e/OS), that option seemed not to be available in Etar, and I successfully used some applications from F-Droid to set up an offline calendar (see Calendar : any offline option? - #9 by sonyxa2).
I am pretty glad to see that now it is possible to do it within /e/OS (although the problem could be easily overcome thanks to F-Droid). But just to add a confirmation for @adamj, yes, it is possible to have an /e/OS phone without any cloud syncronisation.