Time Zone

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The following function portably returns a timezone string in the form +hhmm or -hhmm. One cannot use os.date("%z") as the format of its return value is non-portable; in particular, Windows systems don't use the C99 semantics for strftime(). The following code should portably produce a timezone string for the current local time.

NOTE: the following only computes the timezone offset for "now", which differs from os.date("%z") which can handle times in the past or future, taking daylight savings time into account. Alternatively, you can use get_timezone_anystamp(ts) below

-- Compute the difference in seconds between local time and UTC.
local function get_timezone()
  local now = os.time()
  return os.difftime(now, os.time(os.date("!*t", now)))
timezone = get_timezone()

-- Return a timezone string in ISO 8601:2000 standard form (+hhmm or -hhmm)
local function get_tzoffset(timezone)
  local h, m = math.modf(timezone / 3600)
  return string.format("%+.4d", 100 * h + 60 * m)
tzoffset = get_tzoffset(timezone)

--[[ debugging
for _, tz in ipairs(arg) do
  if tz == '-' then
    tz = timezone
    tz = 0 + tz
  print(tz, get_tzoffset(tz))

-- return the timezone offset in seconds, as it was on the time given by ts
-- Eric Feliksik
local function get_timezone_offset(ts)
	local utcdate   = os.date("!*t", ts)
	local localdate = os.date("*t", ts)
	localdate.isdst = false -- this is the trick
	return os.difftime(os.time(localdate), os.time(utcdate))

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Last edited January 21, 2013 1:18 pm GMT (diff)