.TH computedate "" "" "/rdb Command"
.PC "Add given number of days to a date"
\fBcomputedate \fIdate  days\fR
.PP
.II "date^arithmetic"
.II "arithmetic^date"
The command
.B computedate
adds days to a date, and displays the new date.
.I date
must be the format \fIYYMMDD\fR;
.I days
can be negative or positive.
.B computedate
displays the new date in the same format.
This is the best format for entering dates in a column
because they sort correctly and take only six characters.
.PP
Internally,
.B computedate
converts the date to
.BR julian ,
adds days, then converts the date back to
.BR gregorian .
Suppose we want to know the date 45 days from March 31, 1989.
We can type:
.DM
	computedate 890331 45
.DE
.PP
This command returns the string:
.DM
	890515
.DE
.PP
That is May 15, 1989.
.PP
Another trick is to combine the
.B todaysdate
and the
.B computedate
programs.
.B todaysdate
gives the date of today in the preferred format of \fIYYMMDD\fR.
For example, the following command shows us the date 45 days from today:
.DM
	computedate `todaysdate` 45
.DE
.SH "Data Validation"
You can use
.B computedate
to validate a date.
If you add zero days to a date and get a different date, then
the original date was not valid.
For example, let's test the 32nd day of January 1989:
.DM
	computedate 890132 0
.DE
.PP
This returns the string
.B 890201
or February 1, 1989.
As we expected, we received a different date.
We can test this with the following shell program:
.DM
	if test $DATE = `computedate $DATE 0`
	then
		echo OK
	else
		echo Invalid DATE
	fi
.DE
.SH "See Also"
.B
gregorian,
julian,
todaysdate
.R