Can’t add Milton.edu feed

problem
#1

Feed seems to be fine if I open it in Chrome.

https://www.milton.edu/feed/

#2

invalid certificate.

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.milton.edu%2Ffeed%2F

#3

Since I can’t fix their feed, is there a workaround?