This app would replace calendar event notifications, providing snooze functionality and notifications persistence.
IMPORTANT: please disable any Android power optimizations for this app
NOTE: app in existing state hit a dead-end and will no be further developed or maintained, as it needs significant re-design of the concept to meet demands and requirements of the new API levels.
Also: If you are having repeating / duplicate event notifications - disable manual calendar rescan.
Also: If you are having events firing at wrong time - disable manual calendar rescan. (If still happening - that's an issue with your device or calendar sync provider).