CsvHelper changing how dates and times are output

You can set it globally per type using TypeConverterOptionsFactory.

void Main()
{
    using (var stream = new MemoryStream())
    using (var reader = new StreamReader(stream))
    using (var writer = new StreamWriter(stream))
    using (var csv = new CsvWriter(writer))
    {
        var options = new TypeConverterOptions
        {
            Format = "o"
        };
        TypeConverterOptionsFactory.AddOptions<DateTime>(options);

        csv.WriteField(DateTime.Now);
        csv.NextRecord();
        writer.Flush();
        stream.Position = 0;

        reader.ReadToEnd().Dump();
    }
}

Output:

2016-09-19T11:01:41.5507054-05:00

With newer version (12.1.2) of CsvHelper, it can be archived 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);

For me it was easier to use CsvHelper attributes on class.

[Format("yyyy-MM-dd")] did the job for datetime formatting.

namespace BarNamespace
{
    using System;
    using CsvHelper.Configuration.Attributes;

    public class Foo
    {
        [Name("ColumnName1")]
        [Format("yyyy-MM-dd")]
        public DateTime Date { get; set; }

        [Name("ColumnName2")]
        public string Col2{ get; set; }

        [Name("ColumnName3")]
        public long Col3{ get; set; }
    }
}