Good to see that this is being worked on; I just tried the plugin and started by adding a rule for the duration of the winter holidays, after which the ‘projected dates/times’ (where there was a ‘some problems’ warning even before I started) included a couple of dates for the year 1971, but hardly any for the dates I chose. This is a WordPress with German locale and “j. F Y” and “G:i” date/time formats. I understand the problems you are facing with the code. Resorting to shortcodes for now (which isn’t really a problem).
