It has a solar cell that keeps the internal battery charged. Even room light will do it. It's nice not having to worry about replacing watch batteries!
I had a Citizen EcoDrive chronograph with a perpetual calendar (it's smart enough to know which months have 28/29/30/31 days for years on end) that I was wearing last. Trouble is, I messed it up when I was changing to DST and didn't want to dig out the manual to reset the day and date properly. Then realized I didn't really need it anymore, so it's in a drawer now.
Citizen even has EcoDrive versions now that can listen to WWVB - http://www.nist.gov/...88/grp40/wwvb.cfm - , etc., to reset themselves to the atomic clocks - e.g. http://princetonwatc...op/JY0005-50E.asp Look at all them tiny dials! ;-)
Cheers,
Scott.