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

The previous post, “Getting The Start and End Of The Next Quarter In C# & .NET” looked at how to get the start and end of the next quarter, given a DateOnly.

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

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

Start of the current year

The start of the current year is simply the first day of the year.

A simple algorithm is the following:

1. Get the current year
2. Assume the first date is the 1^st
3. Assume the first month is January
4. Create a new DateOnly with these parameters

So, given a date such as 5 July 2025, the start of the current year is 1 January 2025.

The code is as follows:

public static DateOnly StartOfCurrentYear(this DateOnly date)
{
  //Create a new dateonly using the current year, first day, and first month (Jan)
  return new DateOnly(date.Year, 1, 1);
}

We then write some tests to verify our functionality.

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

End of the current year

The end of the current year, by definition, is the last day of the last month of the current year.

A simple algorithm is as follows:

1. Get the start of the year of the given DateOnly, using the GetStartOfCurrentYear() method above.
2. Get the current year
3. Increment the current year
4. Create a new DateOnly with these parameters
5. Subtract a day

So, given a date, 6 July 2025:

• Start of the year will be 1 Jan 2025
• The current year is 2025
• Increment this to 2026
• Create a new DateOnly, 1 Jan 2026
• Subtract a day, 31 Dec 2025

The code is as follows:

/// <summary>
/// Returns the end of the current year
/// </summary>
/// <param name="date"></param>
/// <returns></returns>
public static DateOnly GetEndOfCurrentYear(this DateOnly date)
{
  // Get the start of the current year
  var startOfCurrentYear = date.GetStartOfCurrentYear();
  // Push to the next year
  var startOfNextYear = new DateOnly(startOfCurrentYear.Year + 1, 1, 1);
  // Advance to end of current year
  return startOfNextYear.AddDays(-1);
}

We then write some tests to verify the functionality.

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

The tests should pass successfully.

TLDR

In this post, we have written methods to compute:

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

The code is in my GitHub.

Happy hacking!