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2017-11-03T21:21:43-0400That is the universal format used by most online: year-month-day-hour-minute-second. Sitemap.xml uses this format.
Page level, it's most convenient to use meta tags.
<meta name='last-modified' content='2017-11-03T21:21:43-0400'>
My server returns 304s "Not Modified" for the ones I didn't change.The server can only do this with fully static content. If you've got anything else--even so much as an included php navigation header--the 304 response will no longer be sent, even if the page as such has not been modified in years.