Identical issue, same behaviour. Also running PHP 8, IPS 4.7.1 with all patches (was not happening with 4.7.0). Started this morning.
Note: I can see individual calendars and add entries but not view the main/summary calendar page (/calendar) – that throws up exactly the same errors as described by @Foolboy.
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Error: Call to undefined method DateTime::getDateInformation() (0)
#0 /applications/calendar/sources/Event/Event.php(363): IPS\calendar\_Event::_findOccurances(Object(IPS\calendar\Date), NULL, Object(IPS\calendar\Date), Object(IPS\calendar\Date), Array)
#1 /applications/calendar/sources/Event/Event.php(2127): IPS\calendar\_Event->findOccurrences(Object(IPS\calendar\Date), Object(IPS\calendar\Date))
#2 /applications/calendar/widgets/upcomingEvents.php(153): IPS\calendar\_Event::retrieveEvents(Object(IPS\calendar\Date), Object(IPS\calendar\Date), NULL, 3, false)
#3 /system/Widget/Widget.php(833): IPS\calendar\widgets\_upcomingEvents->render()
#4 /system/Widget/Widget.php(888): IPS\_Widget->_render()
#5 /system/Theme/Theme.php(885) : eval()'d code(18162): IPS\_Widget->__toString()
#6 /system/Theme/SandboxedTemplate.php(61): IPS\Theme\class_core_front_global->widgetContainer('sidebar', 'vertical')
#7 /system/Theme/Theme.php(885) : eval()'d code(15399): IPS\Theme\_SandboxedTemplate->__call('widgetContainer', Array)
#8 /system/Theme/SandboxedTemplate.php(61): IPS\Theme\class_core_front_global->sidebar('right')
#9 /system/Theme/Theme.php(885) : eval()'d code(6746): IPS\Theme\_SandboxedTemplate->__call('sidebar', Array)
#10 /system/Theme/SandboxedTemplate.php(61): IPS\Theme\class_core_front_global->globalTemplate('ee2f54666441442...', '\n\n<div class='i...', Array)
#11 /system/Dispatcher/Dispatcher.php(173): IPS\Theme\_SandboxedTemplate->__call('globalTemplate', Array)
#12 /system/Dispatcher/Standard.php(113): IPS\_Dispatcher->finish()
#13 /system/Dispatcher/Front.php(625): IPS\Dispatcher\_Standard->finish()
#14 /system/Dispatcher/Dispatcher.php(155): IPS\Dispatcher\_Front->finish()
#15 /index.php(13): IPS\_Dispatcher->run()
#16 {main}
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