NLog does not create the log file until the first log is written. It sounds like you would like your log file named based on the start time of your application (or at least have both the trace log and the error log have the same time-based name so you can easily correlate them). If you don't mind a code approach, you could set a time value in the GlobalDiagnosticsContext and then use the GlobalDiagnosticsContext to name your log files. Something like this should work...
In your program, probably in some code that executes at startup, do something like this:
NLog.GlobalDiagnosticsContext.Set("LogFileBase", DateTime.Now.ToString("yyyyMMdd_HHmmss"));
In your NLog.config file, name your file like this:
fileName=${gdc:LogFileBase}.txt
Now the names of your log files will both be based on the start time of the application.