Add clock-skew script, datetime library

nselib/datetime.lua

@@ -0,0 +1,33 @@
+--- Functions for dealing with dates and timestamps
+--
+-- @copyright Same as Nmap--See https://nmap.org/book/man-legal.html
+-- @class module
+-- @name datetime
+-- @author Daniel Miller
+
+local stdnse = require "stdnse"
+local os = require "os"
+local math = require "math"
+_ENV = stdnse.module("datetime", stdnse.seeall)
+
+--- Record a time difference between the scanner and the target
+--
+-- The skew will be recorded in the host's registry for later retrieval and
+-- analysis. Adjusts for network distance by subtracting half the smoothed
+-- round-trip time.
+--
+--@param host The host being scanned
+--@param timestamp The target timestamp, in seconds.
+--@param received The local time the stamp was received, in seconds.
+function record_skew(host, timestamp, received)
+  local skew_tab = host.registry.datetime_skew
+  skew_tab = skew_tab or {}
+  -- No srtt? I suppose we'll ignore it, but this could cause problems
+  local srtt = host.times and host.times.srtt or 0
+  local adjusted = os.difftime(math.floor(timestamp), math.floor(received)) - srtt / 2.0
+  skew_tab[#skew_tab + 1] = adjusted
+  stdnse.debug2("record_skew: %s", adjusted)
+  host.registry.datetime_skew = skew_tab
+end
+
+return _ENV