Fixing out of date TAI UTC tables (missing information on leap seconds)
If your CASA installation is a bit older, it may happen that upon starting CASA the system complains about missing leap second information for TAI_UTC. Messages like:
SEVERE gaincal::MeasTable::dUTC(Double) (file measures/Measures/MeasTable.cc, line 6307Leap second table TAI_UTC seems out-of-date. SEVERE gaincal::MeasTable::dUTC(Double) (file measures/Measures/MeasTable.cc, line 6307)+ Until table is updated (see aips++ manager) times and coordinates SEVERE gaincal::MeasTable::dUTC(Double) (file measures/Measures/MeasTable.cc, line 6307)+ derived from UTC could be wrong by 1s or more.
Leap seconds are inserted when needed by the Bureau International de l'Heure (BIH) based on geodetic data. Usually missing info on this is benign as leap seconds are only sparsely added or subtracted from the time. Even if they do, your typical dataset would not be affected by much, remember that your typical averaging time of visibilities likely exceeds that second (the impact can be more important for VLBI, high time resolution or extremely precise spectroscopic or astrometric data).
So if the leap second table is out of date, it can be fixed by Updating the CASA Data Repository
If you have CASA installations 4.2.2 or older: The Mac OS version will try to update the repository every time CASA is started. In Linux issue the following shell command. The [casa directory] is the place where you have CASA installed.
# in a terminal cd [casa directory]/data rsync -avz rsync://casa-rsync.nrao.edu/casa-data .