Google- Sales start/end date

[Davef]

Where do you put the sale price on a product page so that it shows up in the feed? Are you using one of the Custom fields? And, how do you format it so that it is:
2011-03-01T16:00-08:00/2011-03-03T16:00-08:00

This is the example provided by Google. Thanks for any help.