# Date and Time Definitions

__Date Definitions:__

Modified Julian Date (MJD):

MJD = Julian Date - 2,400,000.5 days

T, the Besselian date, is given by T = 2000.000 + (MJD - 51544.03) / 365.2422

__Time Definitions:__

UT2-UT1 = 0.022 sin(2*pi*T) - 0.012 cos(2*pi*T) - 0.006 sin(4*pi*T) + 0.007 cos(4*pi*T)

where pi = π and T is the date in Besselian years.

- TT = TAI + 32.184 seconds
- since 00h 00m UTC 01 Jan 2017, the cumulative leap second count is:

TAI-UTC = 37 seconds (exactly)

and GPS Time-UTC = 18 seconds

- users may compute Delta T = TT - UT1 using the relation:

Delta T = 32.184 s + (TAI - UTC) - (UT1 - UTC)

where

(TAI - UTC) is the cumulative number of leap seconds

and

(UT1 - UTC) is a standard Earth Orientation product.