diff --git a/scripts/clock-skew.nse b/scripts/clock-skew.nse
new file mode 100644
index 000000000..3c9068e07
--- /dev/null
+++ b/scripts/clock-skew.nse
@@ -0,0 +1,181 @@
+local coroutine = require "coroutine"
+local datetime = require "datetime"
+local formulas = require "formulas"
+local math = require "math"
+local nmap = require "nmap"
+local stdnse = require "stdnse"
+local table = require "table"
+
+description = [[
+Analyzes the clock skew between the scanner and various services that report timestamps.
+
+At the end of the scan, it will show groups of systems that have similar median
+clock skew among their services. This can be used to identify targets with
+similar configurations, such as those that share a common time server.
+]]
+
+---
+-- @output
+-- Host script results:
+-- |_clock-skew: mean: -13s, deviation: 12s, median: -6s
+--
+-- Post-scan script results:
+-- | clock-skew:
+-- |  -6s: Majority of systems scanned
+-- |  3s:
+-- |    192.0.2.5
+-- |_   192.0.2.7 (example.com)
+--
+-- @xmloutput
+-- <elem key="stddev">12.124355652982</elem>
+-- <elem key="mean">-13.0204495</elem>
+-- <elem key="median">-6.0204495</elem>
+
+author = "Daniel Miller"
+
+license = "Same as Nmap--See https://nmap.org/book/man-legal.html"
+
+categories = {"default", "safe"}
+
+-- These scripts contribute clock skews, so we need them to run first.
+-- portrule scripts do not always run before hostrule scripts, and certainly
+-- not before the hostrule is evaluated.
+dependencies = {
+  "http-date",
+  "http-ntlm-info",
+  "imap-ntlm-info",
+  "ms-sql-ntlm-info",
+  "nntp-ntlm-info",
+  "ntp-info",
+  "pop3-ntlm-info",
+  "rfc868-time",
+  "smb-security-mode",
+  "smtp-ntlm-info",
+  "ssl-date",
+  "telnet-ntlm-info",
+}
+
+hostrule = function(host)
+  return host.registry.datetime_skew and #host.registry.datetime_skew > 0
+end
+
+postrule = function()
+  local tmp = nmap.registry.clock_skews and #nmap.registry.clock_skews > 0
+  stdnse.debug1("rule returns %s", tmp)
+  return tmp
+end
+
+local function format_host (host)
+  local name = stdnse.get_hostname(host)
+  if name == host.ip then
+    return name
+  else
+    return ("%s (%s)"):format(host.ip, name)
+  end
+end
+
+local function record_stats(host, mean, stddev, median)
+  local reg = nmap.registry.clock_skews or {}
+  reg[#reg+1] = {
+    ip = format_host(host),
+    mean = mean,
+    stddev = stddev,
+    median = median,
+    -- Allowable variance to regard this a match.
+    variance = host.times.rttvar * 2
+  }
+  nmap.registry.clock_skews = reg
+end
+
+hostaction = function(host)
+  local mean, stddev = formulas.mean_stddev(host.registry.datetime_skew)
+  local median = formulas.median(host.registry.datetime_skew)
+  record_stats(host, mean, stddev, median)
+  local out = {mean = mean, stddev = stddev, median = median}
+  return out, ("mean: %s, deviation: %s, median: %s"):format(
+    stdnse.format_time(mean),
+    stdnse.format_time(stddev),
+    stdnse.format_time(median)
+    )
+end
+
+local function sorted_keys(t)
+  local ret = {}
+  for k, _ in pairs(t) do
+    ret[#ret+1] = k
+  end
+  table.sort(ret)
+  return ret
+end
+
+--- Return a table that yields elements sorted by key when iterated over with pairs()
+--  Should probably put this in a formatting library later.
+--  Depends on keys() function defined above.
+--@param  t    The table whose data should be used
+--@return out  A table that can be passed to pairs() to get sorted results
+function sorted_by_key(t)
+  local out = {}
+  setmetatable(out, {
+    __pairs = function(_)
+      local order = sorted_keys(t)
+      return coroutine.wrap(function()
+        for i,k in ipairs(order) do
+          coroutine.yield(k, t[k])
+        end
+      end)
+    end
+  })
+  return out
+end
+
+postaction = function()
+  local skews = nmap.registry.clock_skews
+
+  local host_count = #skews
+  local groups = {}
+  for i=1, host_count do
+    local current = skews[i]
+    -- skip if we already grouped this one
+    if not current.grouped then
+      current.grouped = true
+      local group = {current.ip}
+      groups[current.mean] = group
+      for j=i+1, #skews do
+        local check = skews[j]
+        if not check.grouped then
+          -- Consider it a match if it's within a the average variance of the 2 targets.
+          -- Use the median to rule out influence of outliers, since these ought to be discrete.
+          if math.abs(check.median - current.median) < (check.variance + current.variance) / 2 then
+            check.grouped = true
+            group[#group+1] = check.ip
+          end
+        end
+      end
+    end
+  end
+
+  local out = {}
+  for mean, group in pairs(groups) do
+    -- Collapse the biggest group
+    if #group > host_count // 2 then
+      out[stdnse.format_time(mean)] = "Majority of systems scanned"
+    elseif #group > 1 then
+      -- Only record groups of more than one system together
+      out[stdnse.format_time(mean)] = group
+    end
+  end
+
+  if next(out) then
+    return sorted_by_key(out)
+  end
+end
+
+local ActionsTable = {
+  -- hostrule: Get the average clock skew and put it in the registry
+  hostrule = hostaction,
+  -- postrule: compare clock skews and report similar ones
+  postrule = postaction
+}
+
+-- execute the action function corresponding to the current rule
+action = function(...) return ActionsTable[SCRIPT_TYPE](...) end