Add the http-date.nse script. Also the function http.parse_date that

parses an HTTP date in any of three possible formats.

http-date.nse gets the Date: header field value from and HTTP server,
then displays it with how much it differs from local time.

scripts/http-date.nse

@@ -0,0 +1,80 @@
+description = [[
+Gets the date from HTTP-like services. Also prints how much the date
+differs from local time. Local time is the time the HTTP request was
+sent, so the difference includes at least the duration of one RTT.
+]]
+
+---
+-- @output
+-- 80/tcp open  http
+-- |_ http-date: Mon, 13 Jul 2009 20:53:46 GMT; -5s from local time.
+
+author = "David Fifield <david@bamsoftware.com>"
+
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+
+categories = {"discovery", "safe"}
+
+require("http")
+require("shortport")
+
+portrule = shortport.port_or_service({80, 443, 631, 8080},
+	{"http", "https", "ipp", "http-alt"})
+
+-- Turn a positive or negative number of seconds into a string in one of the
+-- forms:
+-- 0s
+-- +2s
+-- -4s
+-- +02m38s
+-- -9h12m34s
+-- 5d17h05m06s
+local function format_difftime(t)
+	local s, sign, sec
+
+	if t > 0 then
+		sign = "+"
+	elseif t < 0 then
+		sign = "-"
+	else
+		sign = ""
+	end
+	t = math.abs(t)
+
+	-- Seconds.
+	sec = t % 60
+	s = string.format("%gs", sec)
+	t = math.floor(t / 60)
+	if t == 0 then return sign .. s end
+	-- Minutes.
+	s = string.format("%02dm%02ds", t % 60, sec)
+	t = math.floor(t / 60)
+	if t == 0 then return sign .. s end
+	-- Hours.
+	s = string.format("%dh", t % 24) .. s
+	t = math.floor(t / 24)
+	if t == 0 then return sign .. s end
+	-- Days.
+	s = string.format("%dd", t) .. s
+	return sign .. s
+end
+
+action = function(host, port)
+	-- Get local time in UTC.
+	local request_time = os.time(os.date("!*t"))
+	local response = http.get(host, port, "/")
+	if not response.status or not response.header["date"] then
+		return
+	end
+
+	local date = http.parse_date(response.header["date"])
+	if not date then
+		return
+	end
+
+	-- Should account for estimated RTT too.
+	local diff = os.difftime(os.time(date), request_time)
+
+	return string.format("%s; %s from local time.",
+		response.header["date"], format_difftime(diff))
+end