Age calculation

Age.cs

using System;
using System.Diagnostics;
using System.Globalization;

namespace ToBeDefined
{
    public static class Age
    {
        /// <summary>
        /// Calculates the age given birthday and referenceDate strings in the form "yyyy-MM-dd".
        /// Age is given in years with one decimal place (floored).
        /// The calculation assumes
        /// - age starts at 0 at date of birth
        /// - a gregorian calendar
        /// - birthdates "February 29" have anniversary at "March 1" in non-leap years.
        /// - calculation ignores exact times of date of birth and reference date (assumes 00:00) 
        /// </summary>
        /// <example>Calculate("2020-02-29", "2021-03-01") == 1.0</example>
        /// <example>Calculate("2020-01-01", "2021-03-15") == 1.2 (1 year and 73 days old)</example>
        public static double Calculate(string birthdateString, string referenceDate)
        {
            var dateOfBirth = ParseUtcDate(birthdateString);
            var today = ParseUtcDate(referenceDate);

            if (dateOfBirth > today)
            {
                throw new ArgumentException("Birthdate must not lie in the future.");
            }
            
            var anniversary = CreateAnniversary(today.Year, dateOfBirth);

            double age;
            if (today >= anniversary)
            {
                // anniversary of birthdate already passed this year
                var nextAnniversary = CreateAnniversary(today.Year + 1, dateOfBirth);
                age = (today.Year - dateOfBirth.Year) +
                      (today - anniversary).TotalDays / (nextAnniversary - anniversary).TotalDays;
            }
            else
            {
                // anniversary of birthday not passed this year
                DateTime prevAnniversary = CreateAnniversary(today.Year - 1, dateOfBirth);
                age = (prevAnniversary.Year - dateOfBirth.Year) +
                      (today - prevAnniversary).TotalDays / (anniversary - prevAnniversary).TotalDays;

            }

            return Math.Floor(10 * age) / 10;
        }

        /// <summary>
        /// Parses a given string in the format "yyyy-MM-dd" as a DateTime value in UTC.
        /// </summary>
        private static DateTime ParseUtcDate(string s)
        {
            return DateTime.ParseExact(
                s, 
                "yyyy-MM-dd", 
                CultureInfo.InvariantCulture.DateTimeFormat).AssumeUtc();
        }

        /// <summary>
        /// Creates a UTC date in the given <paramref name="year"/> that represents the anniversary date for
        /// the given birthdate.
        /// For date of births "February 29" the anniversary is assumed to be "March 1" in non leap years.
        /// </summary>
        /// <example>CreateAnniversary(2021, "1983-13-07") ==> "2021-13-07"</example>
        /// <example>CreateAnniversary(2020, "1991-02-29") ==> "2020-02-29"</example>
        /// <example>CreateAnniversary(2021, "1991-02-29") ==> "2021-03-01"</example>
        private static DateTime CreateAnniversary(int year, DateTime dateOfBirth)
        {
            Debug.Assert(dateOfBirth.Kind == DateTimeKind.Utc);

            int month, day;
            if (!DateTime.IsLeapYear(year) && dateOfBirth.Month == 2 && dateOfBirth.Day == 29)
            {
                month = 3;
                day = 1;
            }
            else
            {
                month = dateOfBirth.Month;
                day = dateOfBirth.Day;
            }
            
            return new DateTime(year, month, day).AssumeUtc();
        }

        /// <summary>
        /// Sets kind to UTC (interpreting DateTime as value in UTC).
        /// </summary>
        private static DateTime AssumeUtc(this DateTime me)
        {
            return DateTime.SpecifyKind(me, DateTimeKind.Utc);
        }
        
    }
}

gistfile1.txt

using System;
using System.Diagnostics;
using System.Globalization;
using ToBeDefined;
using XFSpike.Tests;


namespace ToBeDefined
{
    public class AgeCalculatorTest
    {
        static void Main(string[] args)
        {
            var age1 = Age.Calculate("1983-07-13", "1984-07-13");
            Debug.Assert(age1 == 1);
            
            var age2 = Age.Calculate("1983-07-13", "1984-07-12");
            Debug.Assert(age2 >= 0.9 && age2 < 1);

            var age3 = Age.Calculate("1983-07-13", "1984-09-13");
            Debug.Assert(age3 > 1 && age3 <= 1.2);

            var age4 = Age.Calculate("2020-02-29", "2021-02-28");
            Debug.Assert(age4 > 0 && age4 < 1);
            
            var age5 = Age.Calculate("2020-02-29", "2021-03-01");
            Debug.Assert(age5 == 1);

            var age6 = Age.Calculate("2020-02-29", "2020-02-29");
            Debug.Assert(age6 == 0);

            bool thrown7 = false;
            try
            {
                Age.Calculate("2020-02-29", "2020-02-28");
            }
            catch (Exception e)
            {
                thrown7 = true;
            }
            Debug.Assert(thrown7);

            var a = DateTime.Parse("2021-01-01").AddDays(73);
            var age8 = Age.Calculate("2020-01-01", a.ToString("yyyy-MM-dd"));
            Debug.Assert(age8 == 1.2);
        }
    }
}