Getting The Start and End Of The Next Year In C# & .NET

The previous post, “Getting The Start and End Of The Previous Year In C# & .NET”, discussed how to get the start and end dates of the previous year given a DateOnly.

In this post, we will look at how to compute the following:

1. Start of the next year
2. End of the next year

Start of the next year

To get the start of the next year, we can do the following:

1. Get the start of the current year
2. Get the year
3. Increment the year by 1
4. Create a new DateOnly with these parameters

Thus, given a date, 1 July 2025

1. Start of the current year is 1 Jan 2025
2. The year is 2025
3. Increment to 2026
4. Create a new DateOnly

The code is as follows:

/// <summary>
/// Returns the start of the next year
/// </summary>
/// <param name="date"></param>
/// <returns></returns>
public static DateOnly GetStartOfNextYear(this DateOnly date)
{
  // Create a new dateonly using the current year plus one, first day, and first month (Jan)
  return new DateOnly(date.Year + 1, 1, 1);
}

We next create some tests to make sure it works:

[Trait("Year", "Start")]
[Theory]
[InlineData(2025, 4, 2, 2026, 1, 1)]
[InlineData(2025, 12, 31, 2026, 1, 1)]
public void StartOfNextYearTests(int testYear, int testMonth, int testDay, int actualYear, int actualMonth,
  int actualDay)
{
  var testDate = new DateOnly(testYear, testMonth, testDay);
  testDate.GetStartOfNextYear().Should().Be(new DateOnly(actualYear, actualMonth, actualDay));
}

End of the next year

To get the end of the next year, we leverage some of the work that we have done earlier.

1. Get the start of the current year, using the method we have just written, GetStartOfNextYear()
2. With that DateOnly, get the end of that (current) year, using the GetEndOfCurrentYear() method we wrote here.

The code looks like this:

/// <summary>
/// Return the end of the next year
/// </summary>
/// <param name="date"></param>
/// <returns></returns>
public static DateOnly GetEndOfNextYear(this DateOnly date)
{
  // Get the start of the next year
  var startOfNextYear = date.GetStartOfNextYear();
  // Return the end of that year
  return startOfNextYear.GetEndOfCurrentYear();
}

We then create some tests for the same:

[Trait("Year", "End")]
[Theory]
[InlineData(2025, 4, 2, 2026, 12, 31)]
[InlineData(2025, 12, 31, 2026, 12, 31)]
public void EndOfNextYearTests(int testYear, int testMonth, int testDay, int actualYear, int actualMonth,
  int actualDay)
{
  var testDate = new DateOnly(testYear, testMonth, testDay);
  testDate.GetEndOfNextYear().Should().Be(new DateOnly(actualYear, actualMonth, actualDay));
}

If we run our tests, they should pass.

TLDR

In this post, we have written methods to compute:

1. First day of the next year
2. Last day of the next year

The code is in my GitHub.

Happy hacking!