Here’s a free site you can use to back up all your tweets on Google Calendar so you never have to blame Twitter itself for not having back-up options:
Twistory, is a new Twitter-app site that lets you back-up your valuable tweets on Google Calendar through Google API. It’s simple to use and all you have to do is enter your Twitter username to get started. Of course, if you use your Google Calendar for other purposes, we suggest you to make a new Twitter-backup account for Google Calendar so it stays separate.
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