past tense - "When is the last time...?" OR "When was the last time...?"

I usually say "When was the last time...?", but I came across this conversation. So I wonder which one is correct: is, was or either.

Henry: I was on the phone for an hour.
Ron: Who were you talking to for an hour?
Henry: Sam, an old friend from college.
Ron: When is the last time you talked to him?
Henry: At college graduation!
Ron: Wow! That was 30 years ago!
Henry: We had a lot of catching up to do.

Source: http://www.talkenglish.com/conversation/practice/catching-up-with-a-friend

Answer

Both are correct, although there's a bit of room for discussion.

The last time you spoke to him is obviously in the past, but it is in the past now. So when is the last time is correct.

However, the last time you spoke to him was a time before now. Therefore, When was the last time is also correct.

When was the last time is presently used much more often, as this nGram view shows. Interestingly, neither shows up much before 1960, when what was the last time was more popular, and what is the last time was non-existent.