Serilog RollingFile

Try below:

 var log = new LoggerConfiguration()
          .MinimumLevel.Debug()
          .WriteTo.File(@"f:\log\log.txt", rollingInterval: RollingInterval.Day) 
          .CreateLogger();

The log file name will be automatically log-20150819.txt etc. You do not need to specify the date.Old files will be cleaned up as per retainedFileCountLimit – default is 31.