With newer version (12.1.2) of CsvHelper, it can be achieved by using TypeConverterOptionsCache
var options = new TypeConverterOptions { Formats = new[] { "MM/dd/yyyy" } };
csvWriter.Context.TypeConverterOptionsCache.AddOptions<DateTime>(options);
Output date
08/24/1991
Version 20 moved TypeConverterOptionsCache from Configuration to Context. So the above becomes
var options = new TypeConverterOptions { Formats = new[] { "MM/dd/yyyy" } };
csvWriter.Context.TypeConverterOptionsCache.AddOptions<DateTime>(options);
csvWriter.Context.TypeConverterOptionsCache.AddOptions<DateTime?>(options);