There are many varieties, according to the value of
type as defined
an integer corresponding to a time, in a way that depends on
the type of time (see “Details”).
a string indicating the time zone, used only for unix and matlab times, since GPS times are always referenced to the UTC timezone.
"unix" employs Unix times, measured in seconds since the start
of the year 1970.
"matlab" employs Matlab times, measured in days since what
MathWorks  calls “January 0, 0000” (i.e.
ISOdatetime(0, 1, 1, 0,
0, 0) in R notation).
"gps" employs the GPS convention. For this,
t is a
two-column matrix, with the first column being the the GPS "week"
(referenced to 1999-08-22) and the second being the GPS "second" (i.e. the
second within the week). Since the GPS satellites do not handle leap
seconds, the R-defined
.leap.seconds is used for corrections.
"argo" employs Argo times, measured in days since the start of
the year 1900.
"ncep1" employs NCEP times, measured in hours since the start
of the year 1800.
"ncep2" employs NCEP times, measured in days since the start of
the year 1. (Note that, for reasons that are unknown at this time, a simple
R expression of this definition is out by two days compared with the UDUNITS
library, which is used by NCEP. Therefore, a two-day offset is applied. See
"sas" employs SAS times, indicated by
origin at the start of 1960.
"spss" employs SPSS times, in seconds after 1582-10-14.
"yearday" employs a convention in which
t is a
two-column matrix, with the first column being the year, and the second the
yearday (starting at 1 for the first second of January 1, to match the
convention used by Sea-Bird CTD software).
POSIXct time vector.
 Matlab times: http://www.mathworks.com/help/matlab/ref/datenum.html
 NCEP times: https://www.esrl.noaa.gov/psd/data/gridded/faq.html#3
 problem with NCEP times: https://github.com/dankelley/oce/issues/738
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