This is a function (with supporting code) that attempts to verify a format string for Lua's `os.date` function. It receives the format string as its sole argument and returns `true` plus a formatted date string if it's valid or `false` plus an error message if its invalid. This implementation is designed for World of Warcraft's embedded Lua, but…

VerifyDateFormat.lua

local verifyDateFormat
do
	local specifiers = { -- ANSI C strftime conversion specifiers (usable on Windows and Mac)
		a = true, A = true, b = true,
		B = true, c = true, d = true,
		H = true, I = true, j = true,
		m = true, M = true, p = true,
		S = true, U = true, w = true,
		W = true, x = true, X = true,
		y = true, Y = true, Z = true,
		["%"] = true,
	}

	local specifierPattern

	if IsWindowsClient() then -- Windows only adds one specifiers and one optional flag to the standard set.
		specifiers.z = true
		specifierPattern = "%%#?(%S)"
	elseif IsMacClient() then -- Mac adds specifiers from POSIX, Olson's timezone package, GNU C Library and C99. It also adds several optional flags.
		local S = specifiers
		S["+"], S.B, S.C = true, true, true
		S.D, S.e, S.F = true, true, true
		S.G, S.g, S.h = true, true, true
		S.k, S.l, S.n = true, true, true
		S.O, S.R, S.r = true, true, true
		S.s, S.t, S.T = true, true, true
		S.u, S.V, S.v = true, true, true
		S.X, S.y, S.z = true, true, true
		specifierPattern = "%%[_0%-%^]?%d*[EO]?(%S)"
	else
		error("Client not recognised as Windows or Mac. Something has gone horribly wrong!")
	end

	local invalidSpecifiers = {}
	function verifyDateFormat(input)
		local numInvalidSpecifiers = 0
		for specifier in input:gmatch(specifierPattern) do
			if not specifiers[specifier] then
				numInvalidSpecifiers = numInvalidSpecifiers + 1
				invalidSpecifiers[numInvalidSpecifiers] = s
			end
		end

		if numInvalidSpecifiers == 0 then
			return pcall(date, input) -- If it looks valid, try to call date() with it.
		else
			return false, format(L["Invalid date specifier(s): %s"], tconcat(invalidSpecifiers, ", ", 1, numInvalidSpecifiers))
		end
	end
end