I checked
https://gtalug.org/, and it was definitely on 13 April before, as I did copy & paste.
I guess somebody changed it afterwards.
Yep, we had a typo, fixed when I noticed it.
I expect it's also what's propagating to the iCal file; not sure yet how to get that to update itself.
Actually, looking at <
http://gtalug.org/gtalug.ics>, it *is* updated to 2017-04-10. Why my calendar still thinks it's on the 13th isn't the fault of what the web site is publishing anymore...