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A really helpful feature. Let's hope it's permanent.
[added] Also found an old page with a cache date of 25 February, so definitely retrospective.[/added]
If you see a crawl date from more than a month ago, you're probably checking out a site that handles If-Modified-Since well. You can probably ask Daniel or Matt from Google for more info about it if you see them at SES-San Jose next week.
always used my own time/date stamp in "html comments"
I don't go as far as that (maybe I should), but I have a html comment tag with a last-modified date in my templates and php includes, so I can tell from the source if a cached version is up to date.
If you see a crawl date from more than a month ago, you're probably checking out a site that handles If-Modified-Since well.
Yes, 'probably' but not necessarily. Googlebot grabs about 70% of my pages every week, but there are always some that can go for months without being requested - such as the one with the 25 Feb date I quoted. That had gone through three or more updates since that date.
Currently there is no way of discovering how many similar pages there are in the index without checking every cache. What would be really helpful would be a 'last retrieved' date included in serps as standard, similar to the fresh date one sees now.