format_datetime datetime expected

[Alberto Ramón Portolés]

This works:

ADFActivityRun
| where TimeGenerated > ago(3d)
| extend  duration2 = (End - Start)

This fail:

 ADFActivityRun
| where TimeGenerated > ago(3d)
 extend duration2 = format_datetime((End - Start),'y-M-d h:m:s.fffffff')

Redundant but just in case, fail too

 ADFActivityRun
| where TimeGenerated > ago(3d)
extend duration2 = format_datetime((todatetime(End) - todatetime(Start)),'y-M-d h:m:s.fffffff')

[Alexander Sloutsky]

The function format_datetime() expects datetime value (as error described).
For formating timespans: use https://docs.microsoft.com/en-us/azure/data-explorer/kusto/query/format-timespanfunction

[Alberto Ramón Portolés]

End and Start, is date time
End - Start, is date time

and just in case: todatetime(start) is datetime

[Alexander Sloutsky]

If End is datetime - and Start is datetime, then
End-Start is timespan

[Alberto Ramón Portolés]

ohh, thanks !!