DateTimeExtensions.cs

public static class DateTimeExtensions
{
    private const int Today = 0;
    private const int Yesterday = 1;
    private const int Week = 7;
    private const int Month = 31;

    private const int OneMinute = 60;
    private const int OneHour = 3600;
    private const int OneDay = 86400;

    public static string ToFriendlyDate(this DateTime? date)
    {
        return date.HasValue ? date.Value.ToFriendlyDate() : string.Empty;
    }

    public static string ToFriendlyDate(this DateTime date)
    {
        var elapsedTime = DateTime.UtcNow.Subtract(date);
        var totalDaysElapsed = (int) elapsedTime.TotalDays;
        var totalSecondsElapsed = (int) elapsedTime.TotalSeconds;

        switch (totalDaysElapsed)
        {
            case int _ when totalSecondsElapsed < 0 : return "n/a";
            
            case Today when totalSecondsElapsed < OneMinute: return "just now";

            case Today when totalSecondsElapsed < OneHour:
                return string.Format(new PluralFormatProvider(), "{0:minute;minutes} ago",
                    totalSecondsElapsed.FloorBy(OneMinute));

            case Today when totalSecondsElapsed < OneDay:
                return string.Format(new PluralFormatProvider(), "{0:hour;hours} ago",
                    totalSecondsElapsed.FloorBy(OneHour));

            case Yesterday: return "yesterday";

            case int days when days < Week: return $"{totalDaysElapsed} days ago";

            case int days when days < Month:
                return string.Format(new PluralFormatProvider(), "{0:week;weeks} ago",
                    totalDaysElapsed.CeilingBy(Week));

            default:
                return date.ToString(CultureInfo.InvariantCulture);
        }
    }

    private static double CeilingBy(this int dividend, int divisor) => Math.Ceiling((double) dividend / divisor);

    private static double FloorBy(this int dividend, int divisor) => Math.Floor((double) dividend / divisor);

    private class PluralFormatProvider : IFormatProvider, ICustomFormatter
    {
        public string Format(string format, object arg, IFormatProvider formatProvider) =>
            $"{Convert.ToInt32(arg)} {format.Split(';')[Convert.ToInt32(arg) == 1 ? 0 : 1]}";

        public object GetFormat(Type formatType) => this;
    }
}

---
    
using System;
using FluentAssertions;
using Xunit;

public class DateTimeExtensionsTests
{
    public static object[][] TestData => new[]
    {
        new object[] {DateTime.UtcNow, "just now"},
        new object[] {DateTime.UtcNow.AddMinutes(-1), "1 minute ago"},
        new object[] {DateTime.UtcNow.AddMinutes(-2), "2 minutes ago"},
        new object[] {DateTime.UtcNow.AddMinutes(-60), "1 hour ago"},
        new object[] {DateTime.UtcNow.AddMinutes(-120), "2 hours ago"},
        new object[] {DateTime.UtcNow.AddDays(-1), "yesterday"},
        new object[] {DateTime.UtcNow.AddDays(-2), "2 days ago"},
        new object[] {DateTime.UtcNow.AddDays(-7), "1 week ago"},
        new object[] {DateTime.UtcNow.AddDays(-30), "5 weeks ago"}
    };

    [Theory]
    [MemberData(nameof(TestData))]
    public void ToFriendlyDate_ValidDates(DateTime date, string expected)
    {
        var result = date.ToFriendlyDate();

        result.Should().Be(expected);
    }

    [Fact]
    public void ToFriendlyDate_FutureDate_Invalid()
    {
        var result = DateTime.UtcNow.AddMinutes(5).ToFriendlyDate();

        result.Should().Be("n/a");
    }

    [Fact]
    public void ToFriendlyDate_OldDate()
    {
        var date = DateTime.UtcNow.AddMonths(-3);
        var result = date.ToFriendlyDate();

        result.Should().Be(date.ToString(CultureInfo.InvariantCulture));
    }
}