Intervals that represent divisions of a year begin with the start of the year (1 January). YEARV, R445YR, R454YR, and R544YR intervals begin with the first week of the International Organization for Standardization (ISO) year, the Monday on or immediately preceding January th. R445QTR, R454QTR, and R544QTR intervals begin with the st, th, th, and th weeks of the ISO year. MONTH2 periods begin with odd-numbered months (January, March, May, and so on).

Likewise, intervals that represent divisions of a day begin with the start of the day (midnight). Thus, 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.

Intervals that do not nest within years or days begin relative to the SAS date or datetime value 0. The arbitrary reference time of midnight on January 1, 1960, is used as the origin for nonshifted intervals, and shifted intervals are defined relative to that reference point. For example, MONTH13 defines the intervals January 1, 1960, February 1, 1961, March 1, 1962, and so forth, and the intervals December 1, 1959, November 1, 1958, and so on before the base date January 1, 1960.

Similarly, the WEEK2 interval begins relative to the Sunday of the week of January 1, 1960. The interval specification WEEK6.13 defines six-week periods that start on second Fridays, and the convention of counting relative to the period that contains January 1, 1960, indicates the starting date or datetime of the interval closest to January 1, 1960, that corresponds to the second Fridays of six-week intervals.

Intervals always begin on the date or datetime defined by the base interval name, the multiplier, and the shift value. The end of the interval immediately precedes the beginning of the next interval. However, an interval can be identified by any date or datetime value between its starting and ending values, inclusive. See the section Alignment of SAS Dates for more information about generating identifying dates for intervals.