"unix" handles Unix times, measured in seconds since the start
of the year 1970.
"matlab" handles Matlab times, measured in days since what
MathWorks (reference 1) calls ``January 0, 0000'' (i.e. ISOdatetime(0, 1, 1, 0,
0, 0) in R notation).
"gps" handles the Global Positioning System convention. The
scheme is complicated, owing to hardware limitations of
GPS satellites. As illustrated in Example 3, t may be
a matrix with either 2 or 3 columns. In the 2-column format, the
first column holds the number of weeks after 1999-08-22, modulo
1024 (approximately 19.6 years), and the second column (here
and also in the 3-column format) holds the number of seconds
in the referenced week, with leap seconds being handled with
the leap parameter. The modulo calculation is required
because GPS satellites dedicate only 10 bits to the
week number. The resultant ambiguity (e.g. a rollover in
2019-04-07) is addressed in the 3-column format, in which the
last column holds the number of 1024-week rollover events
since 1980-01-06. Users should set this column to 0 for times
prior to 1999-08-22, to 1 for later times prior
to 2019-04-07, to 2 for later times prior to 2038-11-21,
etc. However, there will be an exception to this rule,
when satellites start dedicating 12 bits to the week value.
For such data, the third column will need to be 0 for all times
prior to 2137-01-06.
"argo" handles Argo times, measured in days since the start of
the year 1900.
"excel" handles Excel times, measured in days since the start of
the year 1900. (Note that excel incorrectly regards 1900 as a leap year,
so 1 day is subtracted from t unless the time is less than or equal
to 1900 Feb 28. Note that NA is returned for the day 60, which is
what excel codes for "Feb 29, 1900", the non-existing day that excel
accepts.
"ncep1" handles NCEP times, measured in hours since the start
of the year 1800.
"ncep2" handles 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
references 2 and 3.)
"sas" handles SAS times, indicated by type="sas", have
origin at the start of 1960.
"spss" handles SPSS times, in seconds after 1582-10-14.
"yearday" handles 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).
"epic" handles a convention used in the EPIC software library,
from the Pacific Marine Environmental Laboratory, in which t is a
two-column matrix, with the first column being the julian Day (as defined in
julianDay(), for example), and with the second column being the
millisecond within that day. See reference 4.
"vms" handles a convention used in the VMS operating system and
for Modified Julian Day, in which t is the number of seconds
past 1859-11-17T00:00:00 UTC. See reference 5.