On iMac
In Preferences please check that your "Date & Time" settings are as below:
Also make sure that your Timezone is set correctly
On iOS (iPhone / iPad)
To save battery life, a number of iPhone/iPad users turn off "Setting Time Zone", however this is needed to automatically update to Daylight Saving.
To address this:
If you go to Settings -> Privacy -> Location Services -> System Services > Setting Time Zone - this should be turned ON - see below
IMPORTANT: Having made these changes remove the feed from your calendar and then re-add it.
Hopefully this will resolve the problem for you but if not please do get in touch.