Yes please! My world has an alternate calendar so being able to link them would be very helpful
I have to manually change the date on my frontpage - which my players use for various bits of info - and will have events tied to moon cycles, so this would be really handy.
I like this idea in principle (if I am understanding it correctly) but I don't think it would work for me. In one of my worlds, I have different realms using different calendars. There's still a concept of universal time (world time) and in some ways it would be nice to pin each calendar to the absolute world time but given that there are different day and year lengths in each calendar I think the best you can do is handle it all manually. You could argue that when the calendars differ that much, the realms should be separate World Anvil worlds and maybe I should do that if I want this kind of functionality, but it is a multiverse setting and I also want some interaction between the realms, so that makes me want to keep it all in one place. This timekeeping stuff with multiple calendars is tricky...
This would solve a lot of display issues. And then timelines would use them as default and you could set a different one if it diverges. I know it would be a big change, but I do support it wholeheartedly