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old issue

You can read the long thread here:
http://blog.scoopz.com/2009/01/09/macbook-air-loses-time-when-battery-runs-flat/

Basically your Mac doesn't have an internal clock battery anymore and running the main battery dead will cause this. If you have an ethernet cable you can plug it into a USB adapter and into your Mac Book Air and then when you wake it up it'll automatically sync to whatever time server you have set in prefs and skip the whole reset clock issue.

Definitely a PAIN. If you are under warranty take back to Apple.
 
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