== Time Fields in DVO ==

mextract and imextract both have fields which represent a time value.  When extracting these fields into vectors, the user has options to control the units and format of the resulting vectors.  The vector is a floating-point value, with a reference date define by the DVO variable $TIMEREF.  The units of the vector may be one of the following values: sec, min, hours, days (note that 'seconds' and 'minutes' invalid and are silently ignored).  

If TIMEREF is not set, it defaults to 1970/01/01,00:00:00.  Some examples of TIMEREF:

{{{
$TIMEREF = 2001/01/01,00:00:00
$TIMEREF = 2009/07/01,00:00:00
}}}

There are two special words for TIMEREF: 'now' and 'today'.  The value of 'now' make the reference the time of the query; the value of 'today' make the reference UT midnight of the day of the query.

Be careful when you set TIMEREF.  if you just do:  $TIMEREF =
2001/01/01, it will treat that as a math expression, and set the
reference to 2001 seconds since 1970/01/01...

There are two special words for TIMEFORMAT: 'jd' and 'mjd'.  With these options, TIMEREF is ignored and the date is returned as a JD or MJD.

The dvo (or mana, etc) function 'ctimes' allows for on-the-fly conversions between human readable time/date values and the formated values.  for example:
{{{
dvo: $TIMEREF = 2009/10/20,00
dvo: echo $TIMEREF
2009/10/20,00

dvo: ctimes -ref 0.0
time: 2009/10/20,00:00:00

dvo: $TIMEFORMAT = sec
dvo: ctimes -abs 2009/10/20,01:00:00
time: 3600.000000

dvo: $TIMEFORMAT = min
dvo: ctimes -abs 2009/10/20,01:00:00
time: 60.000000

dvo: $TIMEFORMAT = hours
dvo: ctimes -abs 2009/10/20,01:00:00
time: 1.000000

dvo: $TIMEFORMAT = days
dvo: ctimes -abs 2009/10/20,01:00:00
time: 0.041667

dvo: $TIMEFORMAT = jd
dvo: ctimes -abs 2009/10/20,01:00:0
time: 2455124.541667

dvo: $TIMEFORMAT = mjd
dvo: ctimes -abs 2009/10/20,01:00:0
}}}