diff --git a/nselib/stdnse.lua b/nselib/stdnse.lua
index 530fd9fcb..681c0a451 100644
--- a/nselib/stdnse.lua
+++ b/nselib/stdnse.lua
@@ -17,6 +17,9 @@ local format = string.format;
 local rep = string.rep
 local concat = table.concat;
 local insert = table.insert;
+local os = os
+local math = math
+local string = string
 
 local nmap = require "nmap";
 
@@ -262,6 +265,85 @@ function string_or_blank(string, blank)
   end
 end
 
+--- Format the difference between times <code>t2</code> and <code>t1</code>
+-- into a string in one of the forms (signs may vary):
+-- * 0s
+-- * -4s
+-- * +2m38s
+-- * -9h12m34s
+-- * +5d17h05m06s
+-- * -2y177d10h13m20s
+-- The string shows <code>t2</code> relative to <code>t1</code>; i.e., the
+-- calculation is <code>t2</code> minus <code>t1</code>.
+function format_difftime(t2, t1)
+  local d, s, sign, yeardiff
+
+  d = os.difftime(os.time(t2), os.time(t1))
+  if d > 0 then
+    sign = "+"
+  elseif d < 0 then
+    sign = "-"
+    t2, t1 = t1, t2
+    d = -d
+  else
+    sign = ""
+  end
+  -- t2 is always later than or equal to t1 here.
+
+  -- The year is a tricky case because it's not a fixed number of days
+  -- the way a day is a fixed number of hours or an hour is a fixed
+  -- number of minutes. For example, the difference between 2008-02-10
+  -- and 2009-02-10 is 366 days because 2008 was a leap year, but it
+  -- should be printed as 1y0d0h0m0s, not 1y1d0h0m0s. We advance t1 to be
+  -- the latest year such that it is still before t2, which means that its
+  -- year will be equal to or one less than t2's. The number of years
+  -- skipped is stored in yeardiff.
+  if t2.year > t1.year then
+    local tmpyear = t1.year
+    -- Put t1 in the same year as t2.
+    t1.year = t2.year
+    d = os.difftime(os.time(t2), os.time(t1))
+    if d < 0 then
+      -- Too far. Back off one year.
+      t1.year = t2.year - 1
+      d = os.difftime(os.time(t2), os.time(t1))
+    end
+    yeardiff = t1.year - tmpyear
+    t1.year = tmpyear
+  else
+    yeardiff = 0
+  end
+
+  local s, sec, min
+  s = ""
+  -- Seconds (pad to two digits).
+  sec = d % 60
+  d = math.floor(d / 60)
+  if d == 0 and yeardiff == 0 then
+    return sign .. string.format("%gs", sec) .. s
+  end
+  s = string.format("%02gs", sec) .. s
+  -- Minutes (pad to two digits).
+  min = d % 60
+  d = math.floor(d / 60)
+  if d == 0 and yeardiff == 0 then
+    return sign .. string.format("%dm", min) .. s
+  end
+  s = string.format("%02dm", min) .. s
+  -- Hours.
+  s = string.format("%dh", d % 24) .. s
+  d = math.floor(d / 24)
+  if d == 0 and yeardiff == 0 then
+    return sign .. s
+  end
+  -- Days.
+  s = string.format("%dd", d) .. s
+  if yeardiff == 0 then return sign .. s end
+  -- Years.
+  s = string.format("%dy", yeardiff) .. s
+  return sign .. s
+end
+
 ---Get the indentation symbols at a given level. 
 local function format_get_indent(indent, at_end)
 	local str = ""
diff --git a/scripts/http-date.nse b/scripts/http-date.nse
index d62fae4a0..0b5120ad8 100644
--- a/scripts/http-date.nse
+++ b/scripts/http-date.nse
@@ -19,12 +19,11 @@ categories = {"discovery", "safe"}
 
 require("http")
 require("shortport")
+require("stdnse")
 
 portrule = shortport.port_or_service({80, 443, 631, 8080},
 	{"http", "https", "ipp", "http-alt"})
 
-local format_difftime
-
 action = function(host, port)
 	-- Get the local date in UTC.
 	local request_date = os.date("!*t")
@@ -39,85 +38,8 @@ action = function(host, port)
 	end
 
 	-- Should account for estimated RTT too.
-	local diff = format_difftime(response_date, request_date)
+	local diff = stdnse.format_difftime(response_date, request_date)
 
 	return string.format("%s; %s from local time.",
 		response.header["date"], diff)
 end
-
--- Turn a positive or negative number of seconds into a string in one of the
--- forms. Signs can of course vary.
--- 0s
--- -4s
--- +2m38s
--- -9h12m34s
--- +5d17h05m06s
--- -2y177d10h13m20s
-format_difftime = function(t2, t1)
-	local d, s, sign, yeardiff
-
-	d = os.difftime(os.time(t2), os.time(t1))
-	if d > 0 then
-		sign = "+"
-	elseif d < 0 then
-		sign = "-"
-		t2, t1 = t1, t2
-		d = -d
-	else
-		sign = ""
-	end
-	-- t2 is always later than or equal to t1 here.
-
-	-- The year is a tricky case because it's not a fixed number of days
-	-- the way a day is a fixed number of hours or an hour is a fixed
-	-- number of minutes. For example, the difference between 2008-02-10
-	-- and 2009-02-10 is 366 days because 2008 was a leap year, but it
-	-- should be printed as 1y0d0h0m0s, not 1y1d0h0m0s. We advance t1 to be
-	-- the latest year such that it is still before t2, which means that its
-	-- year will be equal to or one less than t2's. The number of years
-	-- skipped is stored in yeardiff.
-	if t2.year > t1.year then
-		local tmpyear = t1.year
-		-- Put t1 in the same year as t2.
-		t1.year = t2.year
-		d = os.difftime(os.time(t2), os.time(t1))
-		if d < 0 then
-			-- Too far. Back off one year.
-			t1.year = t2.year - 1
-			d = os.difftime(os.time(t2), os.time(t1))
-		end
-		yeardiff = t1.year - tmpyear
-		t1.year = tmpyear
-	else
-		yeardiff = 0
-	end
-
-	local s, sec, min
-	s = ""
-	-- Seconds (pad to two digits).
-	sec = d % 60
-	d = math.floor(d / 60)
-	if d == 0 and yeardiff == 0 then
-		return sign .. string.format("%gs", sec) .. s
-	end
-	s = string.format("%02gs", sec) .. s
-	-- Minutes (pad to two digits).
-	min = d % 60
-	d = math.floor(d / 60)
-	if d == 0 and yeardiff == 0 then
-		return sign .. string.format("%dm", min) .. s
-	end
-	s = string.format("%02dm", min) .. s
-	-- Hours.
-	s = string.format("%dh", d % 24) .. s
-	d = math.floor(d / 24)
-	if d == 0 and yeardiff == 0 then
-		return sign .. s
-	end
-	-- Days.
-	s = string.format("%dd", d) .. s
-	if yeardiff == 0 then return sign .. s end
-	-- Years.
-	s = string.format("%dy", yeardiff) .. s
-	return sign .. s
-end