|The 2004 transit of Venus over the Sun begins at 5:13:29 am on 8th June, and completes at 11:25:59 am - Universal Time. The time of the 'greatest transit', ie., the instant when Venus passes closest to the Sun's centre is 8:19:44 am.
These times would be valid for an observer at Earth's centre. The actual contact times for any given observer may differ by up to 7 minutes due to the effects of parallax.
The chart above is cast for London at the moment of greatest transit - 8:22:42 am - using Placidus cusps. A table listing local transit times for other major cities around the world can be found here.
For further information see 2004 Transit of Venus by Fred Espenak
The 2012 transit begins at 22:10pm on June 5th and concludes at 04:49am on June 6th. The greatest transit occurs at 1:30 am, June 6th - Universal Time - at 15Gemini 45.