How to get a slowly expanding world border in Minecraft?

Recent Minecraft snapshots added the possibility to set a square world border that players can't pass through. It is used on the MindCrack server to expand the available area very slowly over time. How can I get the same effect in my Minecraft world?

Video with demonstration of what I'm talking about:

Solution 1:

Minecraft allows you to do this directly using different versions of the /worldborder command:

1. Start with /worldborder center ~0 ~0 to center the future border around you.

2. Set the border radius to something small: /worldborder set 30.

3. Make the border grow by a large number of blocks over a large amount of time:

   /worldborder add 1000000 10000000

   will, for example, add 1 million blocks in all directions over 10 million seconds, i.e. 1 block every 10 seconds.

   /worldborder add 1000 1200000

   will increase the world border by one block per day (1200 seconds) for 1000 days.

Repeat steps 2 and 3 (with different numbers) until you're satisfied with the speed.