iCal spec defines UNTIL as either a date or date-time.

Support for rules with UNTIL part set as a date, avoiding coercion into
a Time in the system time zone.

https://tools.ietf.org/html/rfc5545#section-3.3.10

lib/ice_cube/parsers/ical_parser.rb

@@ -37,7 +37,11 @@ def self.rule_from_ical(ical)
         when 'COUNT'
           params[:count] = value.to_i
         when 'UNTIL'
-          params[:until] = Time.parse(value).utc
+          params[:until] = if value =~ /^\d{8}$/
+                             Date.parse(value).to_datetime
+                           else
+                             Time.parse(value)
+                           end.utc
         when 'WKST'
           params[:wkst] = TimeUtil.ical_day_to_symbol(value)
         when 'BYSECOND'