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Date and Time Intervals

Definitions

duration

is an integer representing the difference, in number of days, between any two dates or times or datetimes.

interval

is a unit of measurement that SAS can count within an elapsed period of time, such as DAYS, TENDAYS and SEMIMONTHS.

Syntax

SAS provides date, time, and datetime intervals for counting different periods of elapsed time. You can create multiples of the intervals and shift their starting point. Use them with the INTCK and INTNX functions and with procedures that support numbered lists (such as the PLOT procedure). The form of an interval is

name<multiple><.starting-point>

The terms in an interval have the following definitions:

name

is the name of the interval. See the following table for a list of intervals and their definitions.

multiple

creates a multiple of the interval. Multiple can be any positive number. The default is 1. For example, YEAR2 indicates a two-year interval.

.starting-point

is the starting point of the interval. By default, the starting point is 1. A value greater than 1 shifts the start to a later point within the interval. The unit for shifting depends on the interval, as shown in the following table. For example, YEAR.3 specifies a yearly period from the first of March through the end of February of the following year.

Intervals By Category

Intervals Used with Date and Time Functions

Category	Interval	Definition	Default Starting Point	Shift Period	Example	Description
Date	DAY	Daily intervals	Each day	Days	DAY3	Three-day intervals starting on Sunday
	WEEK	Weekly intervals	Each Sunday	Days (1=Sunday ... 7=Saturday	WEEK.7	Weekly with Saturday as the first day of the week
	WEEKDAY <daysW>	Daily intervals with weekend days treated as part of the preceding weekday. Days identifies the weekend days by number (1=Sunday ... 7=Saturday). By default, days=17.	Each day	Days	WEEKDAY1W WEEKDAY35W	Six-day week with Sunday as a weekend day Five-day week with Tuesday and Thursday as weekend days (W indicates that day 3 and day 5 are weekend days)
	TENDAY	Ten-day intervals (a U.S. automobile industry convention)	First, eleventh, and twenty-first of each month	TENDAY periods	TENDAY4.2	Four ten-day periods starting at the second TENDAY period
	SEMIMONTH	Half-month intervals	First and sixteenth of each month	SEMIMONTH periods	SEMIMONTH2.2	Intervals from the sixteenth of one month through the fifteenth of the next month
	MONTH	Monthly intervals	First of each month	Months	MONTH2.2	February-March, April-May, June-July, August-September, October-November, and December-January of the following year
	QTR	Quarterly (three-month) intervals	January 1 April 1 July 1 October 1	Months	QTR3.2	three-month intervals starting on April 1, July 1, October 1, and January 1
			April 1
			July 1
			October 1
	SEMIYEAR	Semiannual (six-month) intervals	January 1	Months	SEMIYEAR.3	Six-month intervals, March-August and September-February
			July 1
	YEAR	Yearly intervals	January 1	Months
Datetime	Add DT	To any date interval			DTMONTH
					DTWEEKDAY
Time	SECOND	Second intervals	Each second	Seconds
	MINUTE	Minute intervals	Each minute	Minutes
	HOUR	Hourly intervals	Each hour	Hours

Example 3: Calculating a Duration

This program reads the project start and end dates and calculates the duration between them.

data projects;
options nodate pageno=1 linesize=80 pagesize=60;    
   input Projid startdate date9. enddate date9.;
   Duration=enddate-startdate;
   datalines;
398 17oct1997 02nov1997
942 22jan1998 10mar1998
167 15dec1999 15feb2000
250 04jan2001 11jan2001
;

proc print data=projects;
   format startdate enddate date9.;
      title 'Days Between Project Start and Project End';
run;

Output from the PRINT Procedure

Days Between Project Start and Project End run 8 Obs Projid Startdate Enddate Duration 1 398 17OCT1997 02NOV1997 16 2 942 22JAN1998 10MAR1998 47 3 167 15DEC1999 15FEB2000 62 4 250 04JAN2001 11JAN2001 7

Boundaries of Intervals

The SAS System associates date and time intervals with fixed points on the calendar. For example, the MONTH interval represents the time from the beginning of one calendar month to the next, not a period of 30 or 31 days. When you use date and time intervals (for example, with the INTCK or INTNX functions), the SAS System bases its calculations on the calendar divisions that are present. Consider the following examples:

Using INTCK And INTNX

Example	Results	Explanation
mnthnum1= intck( 'month', '25aug2000'd, '05sep2000'd);	mnthnum1=1	The number of MONTH intervals the INTCK function counts depends on whether the first day of a month falls within the period.
mnthnum2= intck( 'month', '01aug2000'd, '31aug2000'd);	mnthnum2=0
next=intnx( 'month', '25aug2000'd,1);	next represents 01sep2000	The INTNX function produces the SAS date value that corresponds to the beginning of the next interval.

Note:   The only intervals that do not begin on the same date in each year are WEEK and WEEKDAY. A Sunday can occur on any date because the year is not divided evenly into weeks.  

Single-Unit Intervals

Single-unit intervals begin at the following points on the calendar:

Single-Unit Intervals

These single-unit intervals		Begin at this point on the calendar
DAY and WEEKDAY		each day
WEEK		each Sunday
TENDAY		the first, eleventh, and twenty-first of each month
SEMIMONTH		the first and sixteenth of each month
MONTH		the first of each month
QTR		the first of January, April, July and October
SEMIYEAR		the first of January and July
YEAR		the first of January

Single-unit time intervals begin as follows:

Single-Unit Time Intervals

These single-unit time intervals		Begin at this point
SECOND		each second
MINUTE		each minute
HOUR		each hour

Multiunit Intervals

Multiunit Intervals Other Than Multiweek Intervals

Multiunit intervals, such as MONTH2 or DAY50, also depend on calendar measures, but they introduce a new problem: the SAS System can find the beginning of a unit (for example, the first of a month), but where does that unit fall in the interval? For example, does the first of October mark the first or the second month in a two-month interval?

For all multiunit intervals except multiweek intervals, the SAS System creates an interval beginning on January 1, 1960, and counts forward from that date to determine where individual intervals begin on the calendar. As a practical matter, when a year can be divided evenly by an interval, think of the intervals as beginning with the current year. Thus, MONTH2 intervals begin with January, March, May, July, September, and November. Consider this example:

Month2 Intervals

SAS statements		Results
howmany1=intck ('month2','15feb2000'd,'15mar2000'd);		howmany1=1
count=intck ('day50','01oct2000'd,'01jan2000'd);		count=1

In the above example, the SAS System counts 50 days beginning with January 1, 1960; then another 50 days; and so on. As part of this count, the SAS System counts one DAY50 interval between October 1, 1998 and January 1, 1999. As an example, to determine the date on which the next DAY50 interval begins, use the INTNX function, as follows:

Using the INTNX Function

SAS statements		Results
start=intnx ('day50','01oct98'd,1);		SAS date value 14200, or Nov 17, 1998

The next interval begins on November 17, 1998.

Time intervals (those that represent divisions of a day) are aligned with the start of the day, that is, midnight. For example, HOUR8 intervals divide the day into the periods 00:00 to 08:00, 8:00 to 16:00, and 16:00 to 24:00 (the next midnight).

Multiweek Intervals

Multiweek intervals, such as WEEK2, present a special case. In general, weekly intervals begin on Sunday, and the SAS System counts a week whenever it passes a Sunday. However, the SAS System cannot calculate multiweek intervals based on January 1, 1960, because that date fell on a Friday, as shown:

Calculating Multi Week Intervals

Therefore, the SAS System begins the first interval on Sunday of the week containing January 1, 1960--that is, on Sunday, December 27, 1959. The SAS System counts multiweek intervals from that point. The following example counts the number of two-week intervals in the month of August, 1998:

Counting Two-Week Intervals

SAS statements		Results
count=intck ('week2','01aug98'D, '31aug98'D);		count=3

To see the beginning date of the next interval, use the INTNX function, as shown here:

Using INTNX to See The Beginning Date of an Interval

SAS statements		Results
begin=intnx('week2','01aug1998'd,1);		"Begin" represents SAS date 14093 or August 02, 1998

The next interval begins on August 16.

Shifted Intervals

Shifting the beginning point of an interval is useful when you want to make the interval represent a period in your data. For example, if your company's fiscal year begins on July 1, you can create a year beginning in July by specifying the YEAR.7 interval. Similarly, you can create a period matching U.S. presidential elections by specifying the YEAR4.11 interval. This section discusses how to use shifted intervals and how the SAS System creates them.

How to Use Shifted Intervals

When you shift a time interval by a subperiod, the shift value must be less than or equal to the number of subperiods in the interval. For example, YEAR.12 is valid (yearly periods beginning in December), but YEAR.13 is not. Similarly, YEAR2.25 is not valid because there is no twenty-fifth month in the two-year period.

In addition, you cannot shift an interval by itself. For example, you cannot shift the interval MONTH because the shifting subperiod for MONTH is one month and MONTH contains only one monthly subperiod. However, you can shift multi-unit intervals by the subperiod. For example, MONTH2.2 specifies bimonthly periods starting on the first day of the second month.

How the SAS System Creates Shifted Intervals

For all intervals except those based on weeks, the SAS System creates shifted intervals by creating the interval based on January 1, 1960, by moving forward the required number of subperiods, and by counting shifted intervals from that point. For example, suppose you create a shifted interval called DAY50.5. The SAS System creates a 50-day interval in which January 1, 1960 is day 1. The SAS System then moves forward to day 5. (Note that the difference, or amount of movement, is 4 days.) The SAS System begins counting shifted intervals from that point. The INTNX function demonstrates that the next interval begins on January 5, 1960:

Using INTNX to Determine When an Interval Begins

SAS statements		Results
start=intnx ('day50.5','01jan1960'd,1);		SAS date value 4, or Jan 5, 1960

For shifted intervals based on weeks, the SAS System first creates an interval based on Sunday of the week containing January 1, 1960 (that is, December 27, 1959), then moves forward the required number of days. For example, suppose you want to create the interval WEEK2.8 (biweekly periods beginning on the second Sunday of the period). The SAS System measures a two-week interval based on Sunday of the week containing January 1, 1960, and begins counting shifted intervals on the eighth day of that. The INTNX function shows the beginning of the next interval:

Using the INTNX Function to Show the Beginning of the Next Interval

SAS statements		Results
start=intnx ('week2.8','01jan1960'd,1);		SAS date value 2, or Jan 3, 1960

You can also shift time intervals. For example, HOUR8.7 intervals divide the day into the periods 06:00 to 14:00, 14:00 to 22:00, and 22:00 to 06:00.

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