"In Dublin's official timezone data, for example, tm_isdst is zero in their summer and one in their winter, the opposite of the norm, but not on RHEL, which "normalizes" the data. If you lived in Dublin and used Fedora, and made an invalid assumption about tm_isdst values, you would not be on time for tea." - from https://www.redhat.com/en/blog/brief-history-mktime