When Would I Use ISO8601 Interval Format?

In SDTMIG v3.4 and SDTM v2.0, formats for Date/Time variables are now specified as "ISO 8601 datetime or interval." You may ask “What's that interval format and when would I use it?”

Most implementers would recognize 2021-10-17T07:44 as a datetime in ISO 8601 format and P14DT7H57M as an ISO 8601 amount of time (duration). However, ISO 8601 intervals are less familiar, even though they're mentioned in the SDTMIG and SENDIG.

The ISO 8601 standard allows an interval of time to be represented in one of three ways

  • With a start datetime and an end datetime separated by a solidus (e.g., YYYY-MM-DDThh:mm:ss/YYYY-MM-DDThh:mm:ss)
  • With a start datetime and an amount of time separated by a solidus (e.g., YYYY-MM-DDThh:mm:ss/PnYnMnDTnHnMnS)
  • With an amount of time and an end datetime separated by a solidus (e.g., PnYnMnDTnHnMnS/YYYY-MM-DDThh:mm:ss)

Events and Interventions have separate variables for start (--STDTC) and end (--ENDTC), so the interval over which an event or intervention can be represented with those two variables. Similarly, if a specimen for a test is collected over an interval, or a test is performed over an interval, that interval can be represented in the separate variables --DTC and --ENDTC.

The interval between a timepoint and a reference timepoint is represented with the two variables: --RFTDTC and --ELTM.

The evaluation interval variable, --EVLINT, usually contains an amount of time, in which case the evaluation interval is assumed to end at the date and time of collection --DTC. However, in rare cases, the evaluation interval does not end at the date of collection. In those cases, the interval may be represented in --EVLINT alone, using an interval format.

A CRF collected the lowest one-hour urine output in a 24- hour period. The urine output was represented in a record in which LBDTC was the start of the one-hour urine collection period and LBENDTC was the end of the one-hour urine collection period. The evaluation interval, LBEVLINT, was the 24-hour period from which the one hour with the lowest urine output was chosen. LBEVLINT used the interval format, YYYY-MM-DDThh:mm:ss/YYYY-MM-DDThh:mm:ss, to represent this 24-hour period.Intervals can also be used to represent an interval of uncertainty, when an actual datetime isn't known, but earliest and latest possible datetimes are known. So, for example, the time of a test could be recorded as "2003-12-15T10:00/2003-12-15T10:30". Recording intervals of uncertainty in a datetime variable is rare since this kind of precision is seldom important enough to be worthwhile. In this example, it would be more common to record a "best guess" time, perhaps "2003-12-15T10:15" or to use a partial datetime, such as "2003-12-15T10".

So, although "or interval" shows up in the ISO 8601 format descriptions for a large number of SDTM variables, the most likely answer to "When would I use ISO8601 interval format" is "rarely."