utils/timestamp.lua
--- source https://github.com/daurnimator/luatz/blob/master/luatz/timetable.lua
-- edited down to just what is needed.

local Public = {}

local floor = math.floor
local strformat = string.format

local function borrow(tens, units, base)
    local frac = tens % 1
    units = units + frac * base
    tens = tens - frac
    return tens, units
end

local function carry(tens, units, base)
    if units >= base then
        tens = tens + floor(units / base)
        units = units % base
    elseif units < 0 then
        tens = tens + floor(units / base)
        units = (base + units) % base
    end
    return tens, units
end

local function is_leap(y)
    if (y % 4) ~= 0 then
        return false
    elseif (y % 100) ~= 0 then
        return true
    else
        return (y % 400) == 0
    end
end

local mon_lengths = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}

-- Number of days in year until start of month; not corrected for leap years
local months_to_days_cumulative = {0}
for i = 2, 12 do
    months_to_days_cumulative[i] = months_to_days_cumulative[i - 1] + mon_lengths[i - 1]
end

local function month_length(m, y)
    if m == 2 then
        return is_leap(y) and 29 or 28
    else
        return mon_lengths[m]
    end
end

local function day_of_year(day, month, year)
    local yday = months_to_days_cumulative[month]
    if month > 2 and is_leap(year) then
        yday = yday + 1
    end
    return yday + day
end

local function leap_years_since(year)
    return floor(year / 4) - floor(year / 100) + floor(year / 400)
end

local leap_years_since_1970 = leap_years_since(1970)

local function normalise(year, month, day, hour, min, sec)
    -- `month` and `day` start from 1, need -1 and +1 so it works modulo
    month, day = month - 1, day - 1

    -- Convert everything (except seconds) to an integer
    -- by propagating fractional components down.
    year, month = borrow(year, month, 12)
    -- Carry from month to year first, so we get month length correct in next line around leap years
    year, month = carry(year, month, 12)
    month, day = borrow(month, day, month_length(floor(month + 1), year))
    day, hour = borrow(day, hour, 24)
    hour, min = borrow(hour, min, 60)
    min, sec = borrow(min, sec, 60)

    -- Propagate out of range values up
    -- e.g. if `min` is 70, `hour` increments by 1 and `min` becomes 10
    -- This has to happen for all columns after borrowing, as lower radixes may be pushed out of range
    min, sec = carry(min, sec, 60) -- TODO: consider leap seconds?
    hour, min = carry(hour, min, 60)
    day, hour = carry(day, hour, 24)
    -- Ensure `day` is not underflowed
    -- Add a whole year of days at a time, this is later resolved by adding months
    -- TODO[OPTIMIZE]: This could be slow if `day` is far out of range
    while day < 0 do
        month = month - 1
        if month < 0 then
            year = year - 1
            month = 11
        end
        day = day + month_length(month + 1, year)
    end
    year, month = carry(year, month, 12)

    -- TODO[OPTIMIZE]: This could potentially be slow if `day` is very large
    while true do
        local i = month_length(month + 1, year)
        if day < i then
            break
        end
        day = day - i
        month = month + 1
        if month >= 12 then
            month = 0
            year = year + 1
        end
    end

    -- Now we can place `day` and `month` back in their normal ranges
    -- e.g. month as 1-12 instead of 0-11
    month, day = month + 1, day + 1

    return {year = year, month = month, day = day, hour = hour, min = min, sec = sec}
end

--- Converts unix epoch timestamp into table {year: number, month: number, day: number, hour: number, min: number, sec: number}
-- @param <number> sec unix epoch timestamp
-- @return <table> {year: number, month: number, day: number, hour: number, min: number, sec: number}
function Public.to_timetable(secs)
    return normalise(1970, 1, 1, 0, 0, secs)
end

--- Converts timetable into unix epoch timestamp
-- @param <table> timetable {year: number, month: number, day: number, hour: number, min: number, sec: number}
-- @return <number>
function Public.from_timetable(timetable)
    local tt = normalise(timetable.year, timetable.month, timetable.day, timetable.hour, timetable.min, timetable.sec)

    local year, month, day, hour, min, sec = tt.year, tt.month, tt.day, tt.hour, tt.min, tt.sec

    local days_since_epoch =
        day_of_year(day, month, year) + 365 * (year - 1970) + -- Each leap year adds one day
        (leap_years_since(year - 1) - leap_years_since_1970) -
        1

    return days_since_epoch * (60 * 60 * 24) + hour * (60 * 60) + min * 60 + sec
end

--- Converts unix epoch timestamp into human readable string.
-- @param <number> secs unix epoch timestamp
-- @return <string>
function Public.to_string(secs)
    local tt = normalise(1970, 1, 1, 0, 0, secs)
    return strformat('%04u-%02u-%02u %02u:%02u:%02d', tt.year, tt.month, tt.day, tt.hour, tt.min, tt.sec)
end

--- Converts unix epoch timestamp into a date string.
-- @param <number> secs unix epoch timestamp
-- @return <string> With data in format YYYY-MM-DD
function Public.to_date_string(secs)
    local tt = normalise(1970, 1, 1, 0, 0, secs)
    return strformat('%04u-%02u-%02u', tt.year, tt.month, tt.day)
end

return Public
