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.

nselib/http.lua

@@ -428,6 +428,63 @@ request = function( host, port, data, options )
 
 end
 
+local MONTH_MAP = {
+  Jan = 1, Feb = 2, Mar = 3, Apr = 4, May = 5, Jun = 6,
+  Jul = 7, Aug = 8, Sep = 9, Oct = 10, Nov = 11, Dec = 12
+}
+
+--- Parses an HTTP date string, in any of the following formats from section
+-- 3.3.1 of RFC 2616:
+-- * Sun, 06 Nov 1994 08:49:37 GMT  (RFC 822, updated by RFC 1123)
+-- * Sunday, 06-Nov-94 08:49:37 GMT (RFC 850, obsoleted by RFC 1036)
+-- * Sun Nov  6 08:49:37 1994       (ANSI C's <code>asctime()</code> format)
+-- @arg s the date string.
+-- @return a table with keys <code>year</code>, <code>month</code>,
+-- <code>day</code>, <code>hour</code>, <code>min</code>, <code>sec</code>, and
+-- <code>isdst</code>, relative to GMT, suitable for input to
+-- <code>os.time</code>.
+function parse_date(s)
+  local day, month, year, hour, min, sec, tz, month_name
+  -- RFC 2616, section 3.3.1:
+
+  -- Handle RFC 1123 and 1036 at once.
+  day, month_name, year, hour, min, sec, tz = s:match("^%w+, (%d+)[- ](%w+)[- ](%d+) (%d+):(%d+):(%d+) (%w+)$")
+  if not day then
+    month_name, day, hour, min, sec, year = s:match("%w+ (%w+)  ?(%d+) (%d+):(%d+):(%d+) (%d+)")
+    tz = "GMT"
+  end
+  if not day then
+    stdnse.print_debug(1, "http.parse_date: can't parse date \"%s\": unknown format.", s)
+    return nil
+  end
+  -- Look up the numeric code for month.
+  month = MONTH_MAP[month_name]
+  if not month then
+    stdnse.print_debug(1, "http.parse_date: unknown month name \"%s\".", month_name)
+    return nil
+  end
+  if tz ~= "GMT" then
+    stdnse.print_debug(1, "http.parse_date: don't know time zone \"%s\", only \"GMT\".", tz)
+    return nil
+  end
+  day = tonumber(day)
+  year = tonumber(year)
+  hour = tonumber(hour)
+  min = tonumber(min)
+  sec = tonumber(sec)
+
+  if year < 100 then
+    -- Two-digit year. Make a guess.
+    if year < 70 then
+      year = year + 2000
+    else
+      year = year + 1900
+    end
+  end
+
+  return { year = year, month = month, day = day, hour = hour, min = min, sec = sec, isdst = false }
+end
+
 get_default_timeout = function( nmap_timing )
   local timeout = {}
   if nmap_timing >= 0 and nmap_timing <= 3 then

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