OBS_NIST FILE (typically 'time.dat')

This file contains clock offsets between observatory clocks (to which
TOAs are referenced) and UTC(NIST).

The first two lines of this file are ignored.  Subsequent lines have
the format:

    col      item
    1-9     MJD
   10-21    offset1 (us)
   22-33    offset2 (us)
   35-35    observatory code
   37-37    flag
 
The difference offset1-offset2 is ObservatoryTime-UTC(NIST).  If the
flag is ' ' (blank), offsets from the two entries closest to the
target MJD are linearly interpolated.  If the flag is 'f', the closest
entry to the target MJD is used without interpolation.

Clock offsets from a given observatory must be in chronological order,
but offsets from different observatories may be intermixed.


NIST_UTC, NIST_BIPM, NIST_PTB, NIST_AT1 FILES

These files contain clock offsets between UTC(NIST) and other
timescales.

The first two lines of these file are ignored.  Subsequent lines have
the format:

    col      item
    1-10    MJD
   11-29    offset

The offset is xxx-UTC(NIST) in nanoseconds, where xxx is UTC,
TT(BIPM), UTC(PTB), or AT1.  Linear interpolation between entries is
performed.


UT1 FILE (typically 'ut1.dat')

This file contains values of UT1-UTC, TAI-UT1, or A1-UT1 (indicated by
setting the KIND variable in the header to 1, 2, or 3, respectively).
The first line of this file is ignored.  Line 2 has the following
format:

    col       item
    1-32     VARFMT, the format of data lines in Fortran notation
   33-34     KIND
   36-42     start JD (integer)
   44-50     end JD (integer, set to a high value to read to end of file)
   52-54     the number of entries per data line
   56-58     interval, in days, between each entry on a data line
   60-74     UNITS (e.g., 1E-3 means milliseconds)

Subsequent lines contain a control string, MJD, and offset entries in
the format described by the VARFMT variable.  If the control string
reads 'END', any further lines of the file are ignored.


LEAP SECOND FILE

File 'leap.sec' in the clkdir directory contains a list of leap
seconds.  One date is stored per line, in ascending order.  Dates are
MJDs.