convertclock dateString ?GMT|{}? ?baseClock?
              Convert  dateString  to an integer clock value (see
              getclock).  This command can parse and convert vir-
              tually  any standard date and/or time string, which
              can include standard time zone mnemonics.  If  only
              a  time  is specified, the current date is assumed.
              If  the  string  does  not  contain  a  time   zone
              mnemonic,  the  local  time zone is assumed, unless
              the GMT argument is specified, in  which  case  the
              clock  value is calculated assuming that the speci-
              fied time is relative to Greenwich Mean  Time.   If
              baseClock  is specified, it is taken as the current
              clock value.  This is useful  for  determining  the
              time on a specific day.

              The character string consists of zero or more spec-
              ifications of the following form:

              time -  A  time  of  day,  which  is  of  the  form
              hh[:mm[:ss]]  [meridian]  [zone] or hhmm [meridian]
              [zone].  If no meridian is specified, hh is  inter-
              preted on a 24-hour clock.

              date - A specific month and day with optional year.
              The acceptable formats are mm/dd[/yy],  yyyy/mm/dd,
              monthname  dd[, yy], dd monthname [yy], and day, dd
              monthname yy.  The  default  year  is  the  current
              year.   If  the year is less then 100, then 1900 is
              added to it.

              relative time - A  specification  relative  to  the
              current  time.   The format is number unit; accept-
              able units are year, fortnight, month,  week,  day,
              hour,  minute  (or  min), and second (or sec).  The
              unit can be specified as a singular or  plural,  as
              in 3 weeks.  These modifiers may also be specified:
              tomorrow, yesterday, today, now, last, this,  next,
              ago.

              The actual date is calculated according to the fol-
              lowing steps.  First, any absolute date and/or time
              is processed and converted.  Using that time as the
              base, day-of-week specifications are added.   Next,
              relative specifications are used.  If a date or day
              is specified, and no absolute or relative  time  is
              given,  midnight is used.  Finally, a correction is
              applied so that the correct hour of the day is pro-
              duced after allowing for daylight savings time dif-
              ferences.

              convertclock ignores case when parsing  all  words.
              The names of the months and days of the week can be
              abbreviated to  their  first  three  letters,  with
              optional  trailing  period.  Periods are ignored in
              any timezone or meridian values.

              Note that convertclock will convert symbolic  time-
              zone  names,  but  these  are  not standardized and
              there are  conflicts  with  various  parts  of  the
              world.   Use  GMT when trying to produce a portable
              time that can then be converted back to  a  numeric
              value.

              The  only  dates  in the range 1902 and 2037 may be
              converted.  Some examples are:

                  convertclock "14 Feb 92"
                  convertclock "Feb 14, 1992 12:20 PM PST"
                  convertclock "12:20 PM Feb 14, 1992"

              This command is provided by Extended Tcl.