Calendar Settings iPad/iPhone
For European, North and South American countries the Gregorian calendar is set automatically by your iPad. For some Eastern countries eg Thailand AC will only work properly if the Gregorian calendar setting is ticked. This setting might also be incorrectly set by mistake.
iPad/iPhone settings -> General -> Language & Region
Calendar settings
Mac Calendar settings
Must be set to Gregorian for AC to work correctly
Calendar Mac settings
Mac Details
Mac choice of calendar