Normally you could use os.time and os.date. But of course the os library isn't available. Most / all of the examples you can find assume you can load libraries which aren't pure lua, which are also not available to a parser.
So you have to do it in pure lua. Below are a couple examples. No guarantee that either is the "best" way.
From Epoch
-- NOTE: will produce incorrect month and day for leap years
local epochYears = {
[1] = {["year"] = "2038", ["first"] = 2145916800},
[2] = {["year"] = "2037", ["first"] = 2114380800},
[3] = {["year"] = "2036", ["first"] = 2082758400},
[4] = {["year"] = "2035", ["first"] = 2051222400},
[5] = {["year"] = "2034", ["first"] = 2019686400},
[6] = {["year"] = "2033", ["first"] = 1988150400},
[7] = {["year"] = "2032", ["first"] = 1956528000},
[8] = {["year"] = "2031", ["first"] = 1924992000},
[9] = {["year"] = "2030", ["first"] = 1893456000},
[10] = {["year"] = "2029", ["first"] = 1861920000},
[11] = {["year"] = "2028", ["first"] = 1830297600},
[12] = {["year"] = "2027", ["first"] = 1798761600},
[13] = {["year"] = "2026", ["first"] = 1767225600},
[14] = {["year"] = "2025", ["first"] = 1735689600},
[15] = {["year"] = "2024", ["first"] = 1704067200},
[16] = {["year"] = "2023", ["first"] = 1672531200},
[17] = {["year"] = "2022", ["first"] = 1640995200},
[18] = {["year"] = "2021", ["first"] = 1609459200},
[19] = {["year"] = "2020", ["first"] = 1577836800},
[20] = {["year"] = "2019", ["first"] = 1546300800},
[21] = {["year"] = "2018", ["first"] = 1514764800},
[22] = {["year"] = "2017", ["first"] = 1483228800},
[23] = {["year"] = "2016", ["first"] = 1451606400},
[24] = {["year"] = "2015", ["first"] = 1420070400},
[25] = {["year"] = "2014", ["first"] = 1388534400},
[26] = {["year"] = "2013", ["first"] = 1356998400},
[27] = {["year"] = "2012", ["first"] = 1325376000},
[28] = {["year"] = "2011", ["first"] = 1293840000},
[29] = {["year"] = "2010", ["first"] = 1262304000},
[30] = {["year"] = "2009", ["first"] = 1230768000},
[31] = {["year"] = "2008", ["first"] = 1199145600},
[32] = {["year"] = "2007", ["first"] = 1167609600},
[33] = {["year"] = "2006", ["first"] = 1136073600},
[34] = {["year"] = "2005", ["first"] = 1104537600},
[35] = {["year"] = "2004", ["first"] = 1072915200},
[36] = {["year"] = "2003", ["first"] = 1041379200},
[37] = {["year"] = "2002", ["first"] = 1009843200},
[38] = {["year"] = "2001", ["first"] = 978307200},
[39] = {["year"] = "2000", ["first"] = 946684800},
[40] = {["year"] = "1999", ["first"] = 915148800},
[41] = {["year"] = "1998", ["first"] = 883612800},
[42] = {["year"] = "1997", ["first"] = 852076800},
[43] = {["year"] = "1996", ["first"] = 820454400},
[44] = {["year"] = "1995", ["first"] = 788918400},
[45] = {["year"] = "1994", ["first"] = 757382400},
[46] = {["year"] = "1993", ["first"] = 725846400},
[47] = {["year"] = "1992", ["first"] = 694224000},
[48] = {["year"] = "1991", ["first"] = 662688000},
[49] = {["year"] = "1990", ["first"] = 631152000},
[50] = {["year"] = "1989", ["first"] = 599616000},
[51] = {["year"] = "1988", ["first"] = 567993600},
[52] = {["year"] = "1987", ["first"] = 536457600},
[53] = {["year"] = "1986", ["first"] = 504921600},
[54] = {["year"] = "1985", ["first"] = 473385600},
[55] = {["year"] = "1984", ["first"] = 441763200},
[56] = {["year"] = "1983", ["first"] = 410227200},
[57] = {["year"] = "1982", ["first"] = 378691200},
[58] = {["year"] = "1981", ["first"] = 347155200},
[59] = {["year"] = "1980", ["first"] = 315532800},
[60] = {["year"] = "1979", ["first"] = 283996800},
[61] = {["year"] = "1978", ["first"] = 252460800},
[62] = {["year"] = "1977", ["first"] = 220924800},
[63] = {["year"] = "1976", ["first"] = 189302400},
[64] = {["year"] = "1975", ["first"] = 157766400},
[65] = {["year"] = "1974", ["first"] = 126230400},
[66] = {["year"] = "1973", ["first"] = 94694400},
[67] = {["year"] = "1972", ["first"] = 63072000},
[68] = {["year"] = "1971", ["first"] = 31536000},
[69] = {["year"] = "1970", ["first"] = 0}
}
local epochMonths = {
[1] = {["month"] = "Dec", ["first"] = 28857600},
[2] = {["month"] = "Nov", ["first"] = 26265600},
[3] = {["month"] = "Oct", ["first"] = 23587200},
[4] = {["month"] = "Sep", ["first"] = 20995200},
[5] = {["month"] = "Aug", ["first"] = 18316800},
[6] = {["month"] = "Jul", ["first"] = 15638400},
[7] = {["month"] = "Jun", ["first"] = 13046400},
[8] = {["month"] = "May", ["first"] = 10368000},
[9] = {["month"] = "Apr", ["first"] = 7776000},
[10] = {["month"] = "Mar", ["first"] = 5097600},
[11] = {["month"] = "Feb", ["first"] = 2678400},
[12] = {["month"] = "Jan", ["first"] = 0}
}
local function convertFromEpoch(epoch)
local year, month, day, hour, minute, seconds
local remainder
for i,j in ipairs(epochYears) do
if epoch > j.first then
year = j.year
epoch = epoch - j.first
break
end
end
if not year then
return
end
for i,j in ipairs(epochMonths) do
if epoch > j.first then
month = j.month
epoch = epoch - j.first
break
end
end
if not month then
return
end
day = string.format("%02d", math.floor(epoch / 86400))
epoch = epoch - (day * 86400)
hour = string.format("%02d", math.floor(epoch / 3600))
epoch = epoch - (hour * 3600)
minute = string.format("%02d", math.floor(epoch / 60))
seconds = string.format("%02d", epoch - (minute * 60))
print(year .. " " .. month .. " " .. day .. " " .. hour .. ":" .. minute .. ":" .. seconds)
end
To Epoch
-- NOTE
-- Expects either:
-- YYYY/MM/DD HH:MM:SS GMT-HH:MM e.g., 2014/06/18 17:03:30 GMT-05:00
-- YYYY-MM-DDTHH:MM:SS.sss-HH:MM e.g., 2011-08-30T09:30:16.768-04:00
local monthSeconds = ({
[1] = 0,
[2] = 2678400,
[3] = 5097600,
[4] = 7776000,
[5] = 10368000,
[6] = 13046400,
[7] = 15638400,
[8] = 18316800,
[9] = 20995200,
[10] = 23587200,
[11] = 26265600,
[12] = 28857600
})
local function convertToEpoch(timeStamp)
if not timeStamp or type(timeStamp) ~= "string" or string.len(timeStamp) < 29 then
return
end
local year = tonumber(string.sub(timeStamp, 1, 4))
local month = tonumber(string.sub(timeStamp, 6, 7))
local day = tonumber(string.sub(timeStamp, 9, 10))
local hour = tonumber(string.sub(timeStamp, 12, 13))
local minute = tonumber(string.sub(timeStamp, 15, 16))
local second = tonumber(string.sub(timeStamp, 18,19))
-- failsafes
if not year or year < 1970 then
return
end
if not month or month > 12 then
return
end
if not day or day > 31 then
return
end
if not hour or hour > 23 then
return
end
if not minute or minute > 59 then
return
end
if not second or second > 59 then
return
end
local epoch
-- calculate years since 1970
local numYears = year - 1970
-- calculate seconds from years
epoch = numYears * 31536000
-- calculate number of leap years
local leapYears = math.floor(numYears / 4)
-- account for days in leap years
epoch = epoch + (leapYears * 86400)
-- determine if there was a recent leap year not accounted for above
local numYearsMod4 = math.mod(numYears, 4)
if numYearsMod4 > 2 then
-- account for the most recent leap year
-- doesn't consider if ~this~ is a leap year
epoch = epoch + 86400
elseif numYearsMod4 == 2 and month > 2 then
-- this is a leap year, and the date is past feb
epoch = epoch + 86400
end
-- add month seconds
epoch = epoch + monthSeconds[month]
-- add day seconds
epoch = epoch + ((day - 1) * 86400)
-- add hour seconds
epoch = epoch + (hour * 3600)
-- add minute seconds
epoch = epoch + (minute * 60)
-- add seconds
epoch = epoch + second
--[[
kludge for leap seconds:
* there were 27 leap seconds added from 1970 to 2016
* does not account for any future leap seconds (if any)
--]]
epoch = epoch + 27
print(epoch)
end
