I like to be able to count down to big milestones in life. Whether it's a big vacation, your birthday or the launch date for your new product.
I created makemeacountdown.com to help out with this as there's nothing that exists on iOS to keep a countdown omnipresent. On the Android platform you can have widgets and there are plenty of countdown widgets that you can throw on your home screen. I wanted the constant reminder for that next big thing.
The way my implementation works is it keeps the countdown in the notification center so that whenever you pull down your notifications pane, you'll see how many days are left until your big event.
The main requirement being that you use Google Calendar and you sync your primary calendar to your phone. Once you've setup a calendar, you'll also see the countdown within your Google Calendar. For best results you should sync your google calendar to your phone at least once daily or have it set to push.
This countdown will appear in every device that you have syncing your Google Calendar syncing to.