Why does relative time, rel_time, sometimes give no value?
Service Level Agreements (SLAs), Response Requirements, Incidents, Analytics
The interval length (the time element reported for each interval) can be reported in two ways:
Relative time takes into account the default response requirements or the service level agreement that is applied to the incident. Relative time uses inc_performance.rel_time and calculates the time of the interval (in seconds) by including only the time that occurred during defined business hours.
Therefore, if an incident is created at 3:00AM and responded to at 8:02AM, if you business hours start at 8:00AM, the relative time for the Created to Initial Response is 120 seconds (namely, 2 minutes). When using relative time, it is possible for no value to be returned.
Absolute time is simply the difference between the two date and time values. For example, date_diff(inc_performance.time_end, inc_performance.time_start). This provides the total amount of time between the two times (in seconds) for the interval.
Therefore, if an incident is created at 3:00AM and responded to at 8:02AM, if you business hours start at 8:00AM, the absolute time for the Created to Initial Response is 18120 seconds (or 5 hours and 2 minutes).
To avoid having no value returned in reports, you could try incorporating absolute time or change the response requirements hours to include more hours of the day.
Response Requirements: You can specify response requirements, work hours, an holidays that are specific to the SLA. The response requirements of the SLA override the default response requirements. This allows you to define more stringent response requirements based on the level of the service being provided by the SLA.
This suggests to me that you might also want to make sure your SLA's are set to the appropriate hours as well. These configurations can also be found under the Service configuration options.