About Date and Time Intervals
Definitions
| duration |
| Tip | Date and datetimes durations can be easily calculated by subtracting the smaller date or datetime from the larger. When dealing with SAS times, special care must be taken if the beginning and the end of a duration are on different calendar days. Whenever possible, the simplest solution is to use datetimes rather than times. |
is a unit of measurement that SAS can count within an elapsed period of time, such as DAYS, MONTHS, or HOURS. The SAS System determines date and time intervals based on fixed points on the calendar and/or the clock. The starting point of an interval calculation defaults to the beginning of the period in which the beginning value falls , which may not be the actual beginning value specified. For instance, if you are using the INTCK function to count the months between two dates, regardless of the actual day of the month specified by the date in the beginning value, SAS treats it as the first of that month.
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
| 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 of seven days | Each Sunday | Days (1=Sunday 7=Saturday) | WEEK.7 | Weekly with Saturday as the first day of the week | |
| WEEKDAY < days W> | Daily intervals with Friday-Saturday-Sunday | Each day | Days | WEEKDAY1W | Six-day week with Sunday as a weekend day | |
| counted as the same day (five-day work week with a Saturday-Sunday weekend). Days identifies the weekend days by number (1=Sunday 7=Saturday). By default, days =17. | WEEKDAY35W | 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 | Ten-day periods | TENDAY4.2 | Four ten-day periods starting at the second TENDAY period | |
| SEMIMONTH | Half-month intervals | First and sixteenth of each month | Semi-monthly 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 | Months | QTR3.2 | Three-month intervals starting on April 1, July 1, October 1, and January 1 | |
| SEMIYEAR | Semiannual (six-months) intervals | January 1 | Months | SEMIYEAR.3 | Six-month intervals, March-August and September-February | |
| YEAR | Yearly intervals | January 1 | Months | |||
| Datetime | Add DT | To any date interval | Midnight of January 1, 1960 | DTMONTH | DTWEEKDAY | |
| Time | SECOND | Second intervals | Start of the day (midnight) | Seconds | ||
| MINUTE | Minute intervals | Start of the day (midnight) | Minutes | |||
| HOUR | Hourly intervals | Start of the day (midnight) | Hours |
Example: 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;
 |
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:
| 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:
| 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 :
| 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:
| 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:
| 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:
Figure 8.2: 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:
| 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:
| 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:
| 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:
| 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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