вычисление времени

SYNOPSIS
     date [-ju] [-r seconds] [-v [+|-]val[ymwdHMS]] ... [+output_fmt]
     date [-jnu] [[[[[cc]yy]mm]dd]HH]MM[.ss]
     date [-jnu] -f input_fmt new_date [+output_fmt]
     date [-d dst] [-t minutes_west]

     -v      Adjust (i.e., take the current date and display the result of the
             adjustment; not actually set the date) the second, minute, hour,
             month day, week day, month or year according to val.  If val is
             preceded with a plus or minus sign, the date is adjusted forwards
             or backwards according to the remaining string, otherwise the
             relevant part of the date is set.  The date can be adjusted as
             many times as required using these flags.  Flags are processed in
             the order given.
             When setting values (rather than adjusting them), seconds are in
             the range 0-59, minutes are in the range 0-59, hours are in the
             range 0-23, month days are in the range 1-31, week days are in
             the range 0-6 (Sun-Sat), months are in the range 1-12 (Jan-Dec)
             and years are in the range 80-38 or 1980-2038.
             If val is numeric, one of either y, m, w, d, H, M or S must be
             used to specify which part of the date is to be adjusted.
             The week day or month may be specified using a name rather than a
             number.  If a name is used with the plus (or minus) sign, the
             date will be put forwards (or backwards) to the next (previous)
             date that matches the given week day or month.  This will not
             adjust the date, if the given week day or month is the same as
             the current one.
             When a date is adjusted to a specific value or in units greater
             than hours, daylight savings time considerations are ignored.
             Adjustments in units of hours or less honor daylight saving time.
             So, assuming the current date is March 26, 0:30 and that the DST
             adjustment means that the clock goes forward at 01:00 to 02:00,
             using -v +1H will adjust the date to March 26, 2:30.  Likewise,
             if the date is October 29, 0:30 and the DST adjustment means that
             the clock goes back at 02:00 to 01:00, using -v +3H will be nec-
             essary to reach October 29, 2:30.
             When the date is adjusted to a specific value that doesn't actu-
             ally exist (for example March 26, 1:30 BST 2000 in the
             Europe/London timezone), the date will be silently adjusted for-
             wards in units of one hour until it reaches a valid time.  When
             the date is adjusted to a specific value that occurs twice (for
             example October 29, 1:30 2000), the resulting timezone will be
             set so that the date matches the earlier of the two times.
             Adjusting the date by months is inherently ambiguous because a
             month is a unit of variable length depending on the current date.
             This kind of date adjustment is applied in the most intuitive
             way.  First of all, date tries to preserve the day of the month.
             If it is impossible because the target month is shorter than the
             present one, the last day of the target month will be the result.
             For example, using -v +1m on May 31 will adjust the date to June
             30, while using the same option on January 30 will result in the
             date adjusted to the last day of February.  This approach is also
             believed to make the most sense for shell scripting.  Neverthe-
             less, be aware that going forth and back by the same number of
             months may take you to a different date.
             Refer to the examples below for further details.
     An operand with a leading plus (`+') sign signals a user-defined format
     string which specifies the format in which to display the date and time.
     The format string may contain any of the conversion specifications
     described in the strftime(3) manual page, as well as any arbitrary text.
     A newline (`\n') character is always output after the characters speci-
     fied by the format string.  The format string for the default display is
     ``+%+''.
     If an operand does not have a leading plus sign, it is interpreted as a
     value for setting the system's notion of the current date and time.  The
     canonical representation for setting the date and time is:
           cc      Century (either 19 or 20) prepended to the abbreviated
                   year.
           yy      Year in abbreviated form (e.g., 89 for 1989, 06 for 2006).
           mm      Numeric month, a number from 1 to 12.
           dd      Day, a number from 1 to 31.
           HH      Hour, a number from 0 to 23.
           MM      Minutes, a number from 0 to 59.
           ss      Seconds, a number from 0 to 61 (59 plus a maximum of two
                   leap seconds).
     Everything but the minutes is optional.
     Time changes for Daylight Saving Time, standard time, leap seconds, and
     leap years are handled automatically.