I could find no FAQ on this so I am reporting this here.  Everyone repeats that %date{ISO8601} will produce ISO8601 format for dates.  Beyond that, the documentation says that the default form of the %date in PatternLayout is ISO8601.

 

This appears to be wrong based on the statement of the ISO8601 standard at http://www.w3.org/TR/NOTE-datetime. 

 

An example should illustrate the difference easily.

 

Sample output of %date{ISO8601}:

 

But if the above elements (down to the fractional second) are to be displayed, the ISO8601 standard says it should be (assuming Pacific Time in this sample)

 

The ISO8601 correct format is *possible* in log4net, of course, but it is not the default.  I don’t know whether this is a bug in log4net or whether the documentation is just out of date or what.  I will leave that for others to sort out but there is a mismatch between the documentation and the behavior.