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

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

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

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

Start of the previous year

To get the start of the previous year, we do the following:

• Get the start of the current year
• Get the current year
• Decrement the current year by one
• Return a new DateOnly with these parameters

Thus, given a date, 2 July 2025

• Start of the year is 1 January 2025
• Current year is 2025
• The previous year is 2024
• The start of the previous year is 1 January 2024

The code is as follows:

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

We then write some tests to verify it works:

[Trait("Year", "Start")]
[Theory]
[InlineData(2025, 4, 2, 2024, 1, 1)]
[InlineData(2025, 12, 31, 2024, 1, 1)]
public void StartOfPreviousYearTests(int testYear, int testMonth, int testDay, int actualYear, int actualMonth,
int actualDay)
{
  var testDate = new DateOnly(testYear, testMonth, testDay);
  testDate.GetStartOfPreviousYear().Should().Be(new DateOnly(actualYear, actualMonth, actualDay));
}

End of the previous year

To get the end of the previous year, we do the following, leveraging some of our earlier work:

1. Get the start of the previous year, using the GetStartOfPreviousYear() method we just wrote.
2. Using the GetEndOfCurrentYear() method we wrote earlier, get the last day of the year

The code is as follows:

/// <summary>
/// Returns the end of the previous year
/// </summary>
/// <param name="date"></param>
/// <returns></returns>
public static DateOnly GetEndOfPreviousYear(this DateOnly date)
{
  // Get the start of the previous year
  var startOfPreviousYear = date.GetStartOfPreviousYear();
  // Return the end of that year
  return startOfPreviousYear.GetEndOfCurrentYear();
}

We then write some tests for this:

[Trait("Year", "End")]
[Theory]
[InlineData(2025, 4, 2, 2024, 12, 31)]
[InlineData(2025, 12, 31, 2024, 12, 31)]
public void EndOfPreviousYearTests(int testYear, int testMonth, int testDay, int actualYear, int actualMonth,
int actualDay)
{
  var testDate = new DateOnly(testYear, testMonth, testDay);
  testDate.GetEndOfPreviousYear().Should().Be(new DateOnly(actualYear, actualMonth, actualDay));
}

TLDR

In this post, we have written methods to compute:

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

The code is in my GitHub.

Happy hacking!