Julian Day, or number of days since January 1, 4713 BCE at noon UTC.
Warning
This simplification is only valid between 1901 and 2099.
To get correct values it is recommended to increase the number of digits to display:
options(digits=12)
References
Danby, J. M. Eqn. 6.16.4 in Fundamentals of Celestial Mechanics, 2nd ed. Richmond, VA: Willmann-Bell, p. 207, 1988.