How can I set the log filename based on the current date?
 

In the Logging options dialog (File | Properties | Logging), a value of

%PROJDIR%\%PROJROOT%\[vbld_FormatDate()].xml

for the Log filename will log to a file in the project path with the filename formatted with the current date in yyyymmdd format. It uses the system script function vbld_FormatDate.

Or, you can call the vbld_FormatDateEx() script function if different formatting is required. For instance,

[vbld_FormatDateEx(Now, "yyyy-mm-d hh-MM-SS")]

See the help for more details.
http://www.visualbuild.com/Manual/sysscriptdatetime.htm
http://www.visualbuild.com/Manual/buildlogging.htm