SofaUtctai Method

public static short Utctai(
	double utc1,
	double utc2,
	ref double tai1,
	ref double tai2
)

Public Shared Function Utctai ( 
	utc1 As Double,
	utc2 As Double,
	ByRef tai1 As Double,
	ByRef tai2 As Double
) As Short

Parameters

utc1  Double

UTC as a 2-part quasi Julian Date (Notes 1-4)

utc2  Double

UTC as a 2-part quasi Julian Date (Notes 1-4)

tai1  Double

TAI as a 2-part Julian Date (Note 5)

tai2  Double

TAI as a 2-part Julian Date (Note 5)

Return Value

1. utc1+utc2 is quasi Julian Date (see Note 2), apportioned in any convenient way between the two arguments, for example where utc1 is the Julian Day Number and utc2 is the fraction of a day.
2. JD cannot unambiguously represent UTC during a leap second unless special measures are taken. The convention in the present function is that the JD day represents UTC days whether the length is 86399, 86400 or 86401 SI seconds. In the 1960-1972 era there were smaller jumps (in either direction) each time the linear UTC(TAI) expression was changed, and these "mini-leaps" are also included in the SOFA convention.
3. The warning status "dubious year" flags UTCs that predate the introduction of the time scale or that are too far in the future to be trusted. See iauDat for further details.
4. The function iauDtf2d converts from calendar date and time of day into 2-part Julian Date, and in the case of UTC implements the leap-second-ambiguity convention described above.
5. The returned TAI1,TAI2 are such that their sum is the TAI Julian Date.

Reference