Include a date argument in your ConversionPattern. From the PatternLayout documentation:
date -
Used to output the date of the logging event in the local time zone.
To output the date in universal time use the %utcdate pattern. The
date conversion specifier may be followed by a date format specifier
enclosed between braces. For example, %date{HH:mm:ss,fff} or %date{dd
MMM yyyy HH:mm:ss,fff}. If no date format specifier is given then
ISO8601 format is assumed (Iso8601DateFormatter).
The date format specifier admits the same syntax as the time pattern
string of the ToString.
For better results it is recommended to use the log4net date
formatters. These can be specified using one of the strings
"ABSOLUTE", "DATE" and "ISO8601" for specifying
AbsoluteTimeDateFormatter, DateTimeDateFormatter and respectively
Iso8601DateFormatter. For example, %date{ISO8601} or %date{ABSOLUTE}.
These dedicated date formatters perform significantly better than
ToString.
answered 2012-02-02T17:18:07.577