Lightning calendar converts single RDATE to multiple and does not consider PERIOD

Hello, I'm trying to create a calendar event with irregular recurrence. Not only is the starting time different, but also the duration. Since AFAIK this is not possible using the interface, I directly edited the .ics to this: RDATE;VALUE=PERIOD:20260506T063000Z/20260506T130000Z,20260507T054500Z/20260507T110000Z I then manually synced the calendar. The starting time is honored, the duration, however, is not. I found the entry in the .ics changed by Thunderbird to this: RDATE:20260506T063000Z RDATE:20260507T054500Z How can I get Thunderbird to honor the different durations? Thanks