# julianday

`function julianday( year, month, day ) --> jd`

## Description

Converts a Gregorian date to a Julian Day number.

## Parameters

year

The year as a number.

month

The month as a number from 1(January) to 12(December).

day

The day as a number from 1 to 31

## Return Values

jd

The corresponding julian day for the given gregorian date.

## Code

```--ZFUNC-julianday-v1
local function julianday( year, month, day ) --> jd
if year < 0 then year = year + 1 end
if ( year > 1582 or
( year == 1582 and ( month > 10 or
( month == 10 and day >= 15 ) ) ) ) then

local k = math.ceil( ( month - 14 ) / 12 )
local a = math.floor( ( year + 4800 + k ) * 365.25 )
local b = math.floor( ( 367 * ( month - 2 - 12 * k ) ) / 12 )
local c = math.floor( ( ( year + 4900 + k ) / 100 ) * 0.75 )

return a + b - c + day - 32075
elseif ( year < 1582 or
( year == 1582 and ( month < 10 or
( month == 10 and day <= 4 ) ) ) ) then

local k = math.floor( ( 14 - month ) / 12 )
local a = math.floor( ( 153 * ( month + ( 12 * k ) - 3) + 2 ) / 5 )
local b = math.floor( ( year + 4800 - k ) * 365.25 )

return a + b + day - 32083;
else
return 0
end
end

return julianday```

## Examples

```local t = require( "taptest" )
local julianday = require( "julianday" )

-- should work like http://aa.usno.navy.mil/data/docs/JulianDate.php at 12:00:00
t( julianday( 1, 1, 1 ), 1721424 )
t( julianday( 333, 1, 27 ), 1842713 )
t( julianday( 1582, 10, 4 ),2299160 )

t( julianday( 1582, 10, 15 ), 2299161 )
t( julianday( 1900, 1, 1 ), 2415021 )
t( julianday( 1990, 1, 1 ), 2447893 )

t( julianday( 2000, 1, 1 ), 2451545 )
t( julianday( 2006, 1, 14 ), 2453750 )
t( julianday( 2010, 3, 25 ), 2455281 )
t( julianday( 2014, 5, 20 ), 2456798 )

t()```