date: add date/time setting capability

[Michael Forney: Moved functionality to setdate, other minor tweaks]

date.1

@@ -3,12 +3,17 @@
 .Os sbase
 .Sh NAME
 .Nm date
-.Nd print date and time
+.Nd print or set date and time
 .Sh SYNOPSIS
 .Nm
 .Op Fl d Ar time
 .Op Fl u
-.Op Cm + Ns Ar format
+.Oo
+.Cm + Ns Ar format |
+.Sm off
+.Ar mmddHHMM Oo Oo Ar CC Oc Ar yy Oc
+.Sm on
+.Oc
 .Sh DESCRIPTION
 .Nm
 prints the date and time according to
@@ -16,7 +21,8 @@ prints the date and time according to
 or
 .Ar format
 using
-.Xr strftime 3 .
+.Xr strftime 3
+or sets the date.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl d Ar time
@@ -25,8 +31,46 @@ Print
 given as the number of seconds since the
 Unix epoch 1970-01-01T00:00:00Z.
 .It Fl u
-Print UTC time instead of local time.
+Print or set UTC time instead of local time.
 .El
+.Pp
+An operand with a leading plus
+.Pq Cm +
+sign signals a user-defined format string using
+.Xr strftime 3
+conversion specifications.
+.Pp
+An operand without a leading plus sign is interpreted as a value
+for setting the system's current date and time. The canonical
+representation for setting the date and time is:
+.Pp
+.Bl -tag -width Ds -compact -offset indent
+.It Ar mm
+The month of the year, from 01 to 12.
+.It Ar dd
+The day of the month, from 01 to 31.
+.It Ar HH
+The hour of the day, from 00 to 23.
+.It Ar MM
+The minute of the hour, from 00 to 59.
+.It Ar CC
+The first two digits of the year (the century).
+.It Ar yy
+The second two digits of the year.
+If
+.Ar yy
+is specified, but
+.Ar CC
+is not, a value for
+.Ar yy
+between 69 and 99 results in a
+.Ar CC
+value of 19. Otherwise, a
+.Ar CC
+value of 20 is used.
+.El
+.Pp
+The century and year are optional. The default is the current year.
 .Sh STANDARDS
 The
 .Nm