Converting a UTC DateTime to string and back again for quality API design in C# / .NET!

A key design pattern when building proper APIs.

Posted on March 14, 2022


🥧 obligatory pie emoji for Pi day pun - all nerds unite!

Recently I had to build an endpoint where a datetime would be passed in from the frontend and would have to be retrieved later. This is a super thing in API design, but I was running into an overload of ways to do it in .NET. I knew by best practices that I should be forming the actual DateTime objects with UTC time, and then serializing them to a string according to the ISO 8601 standard.

The possibilities and combinations of parameters seemed endless, and I was seeing a variety of answers. Should I use:

  • ToString()?
  • Convert.ToDateTime()?
  • DateTime.Parse()?

It was only after some significant trial and error that I got to the final solution as presented here, and it's something I don't remember learning about in the common tutorials about C# and .NET when I was first learning about them.

DateTime to string and back Again, a Hobbit's Tale

I recommend the following. Given a variable of type DateTime:

var myDatetime = DateTime.UtcNow;

DateTime to string

When we want to form an ISO 8601 string from a DateTime, we should use ToString("o"):

var myDatetimeIsoString = myDatetime.ToString("o");
// "2022-03-14T15:21:41.227Z"

Where the "o" format specifier tells C# to use the round-trip format (why this is not default, I don't know!)

string to DateTime

When we want to go back to the exact representation of that string to DateTime, we should use DateTime.Parse:

var myDatetimeAgain = DateTime.Parse(myDatetimeIsoString, null, System.Globalization.DateTimeStyles.RoundtripKind);

Here, null is for the culture, and the third parameter is the 'kind', which of course we want as the RoundtripKind.

In Summary

Using ToString("o") and DateTime.Parse(), you can convert between string to DateTime, over and over again and can be sure you'll never lose information or find them to be unequal! Perfect.

Test It!

Nowadays, I almost feel like I can't write any code without having a test to confirm it. It helps me design my systems and gives me reassurance whenever I do any renames or refactors.

using System;
using System.Diagnostics;

public class Program
    public static void Main()
        var myDateTime = DateTime.UtcNow;
        var myDateTimeIsoString = myDateTime.ToString("o");
        var myDateTimeAgain = DateTime.Parse(myDateTimeIsoString, null, System.Globalization.DateTimeStyles.RoundtripKind);
        // Doesn't throw :)
        Debug.Assert(myDateTime == myDateTimeAgain);

Here's a .NET Fiddle if you want to run this yourself.

This is the Way

Using this method (and our test to confirm it) you are sure your API deals with UTC Datetimes only, and that they can always be properly converted to the ISO 8601 datetime string, and subsequently to xthe correct timezone in your various clients, no matter where (and when 😄) they may be around the world.


Hope you enjoyed this post and that it saves you from scratching your head in the innumerable ways of converting DateTime to string in C# (and vice versa).

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