|EPOCH||D||(Julian) epoch (e.g. 1850D0)|
|sla_DT||D||approximate ET-UT (after 1984, TT-UT1) in seconds|
before AD 979 Stephenson & Morrison's 390 BC to AD 948 model AD 979 to AD 1708 Stephenson & Morrison's AD 948 to AD 1600 model after AD 1708 McCarthy & Babcock's post-1650 modelThe breakpoints are chosen to ensure continuity: they occur at places where the adjacent models give the same answer as each other.
SLALIB --- Positional Astronomy Library