I’ve just pushed version 1.5 and it includes both things you mentioned:
Optional PlaceholderAPI support
If PlaceholderAPI is installed, the plugin now registers a small set of placeholders you can use in scoreboards, menus, holograms, etc.:
%aeternum_day% – current day
%aeternum_year% – current year
%aeternum_season% – current season (using the plugin’s translations)
%aeternum_realm% – curre_
%aeternum_world_name% – tech
config.yml
features:
portals:
frost:
enabled: true # false = no portals to the frozen world
heat:
enabled: true # false = no portals to the underworld
Version: 1.2
Best possible seasons plugin on the market, also it's free and good support from the developer!