I use timeanddate.com probably every day, on average, for work. It's so handy, for both the calendars and the date calculators.
date --date="2 days ago"
cal -3
On the other hand, I admire whoever the operator is for creating a business around such a simple thing.
So it's Bash that is slower ("heavyweight") if you count how many neuron firings it takes and how much these gigantic, slow write heads (fingers on a keyboard) have to move through space as part of the system. Computer scientists haven't been able to make a language with any sort of error recovery because "it would be non-deterministic" but Google (guided by capitalism) was able to see that's what people need. Google search is a programming language/shell.
Maybe the date command can do things like calculate the number of business days between two dates, accounting for U.S. holidays, and add or subtract work days excluding U.S. holidays -- I don't know. But I know it would take more effort than I'm willing to do to figure that out and test it. And I suspect it would involve a few switches and be subject to easy error.
For pulling up the calendar, it's literally a bookmark that brings up a browser tab with the year's calendar, showing U.S. national holidays. Can the cal command show holidays at all? It's unclear.
I think it's a bit sad to discount the value of a clear, usable, and fast GUI over a command-line interface for certain day-to-day tasks. Both tools have their uses.