What's worth writing in about
Three kinds of messages are especially useful: a calculator result that doesn't match what you expected (tell us the page and the dates you entered, and we'll check the math), a calculator you wish existed, or anything about the site that's confusing or broken. General feedback is welcome too.
Response time
DateTimeHQ is run by a small team, so replies aren't instant, but every message is read. If you've reported something that looks like a genuine calculation error, that gets looked at first.