SystemTime wrapper around DateTime for a unit testing hook

SystemTime

// another option? https://blog.burgyn.online/2019/12/06/date-time-provider
using System;

namespace Shared.Core.Implementations
{
    /// <summary>
    /// Testable alternative to DateTime.Now or DateTimeOffset.Now
    /// </summary>
    public static class SystemTime
    {
        private static DateTime? _dateTimeHolder = null;

        /// <summary>
        /// Gets the current local DateTimeOffset
        /// </summary>
        public static DateTime Now => _dateTimeHolder ?? DateTime.Now;

        /// <summary>
        /// Allow overriding the time for testing
        /// </summary>
        /// <param name="fakeDateTime"></param>
        public static void SetNow(DateTime fakeDateTime)
        {
            _dateTimeHolder = fakeDateTime;
        }
        public static void ResetNow()
        {
            _dateTimeHolder = null;
        }

        private static DateTimeOffset? _offsetHolder = null;

        /// <summary>
        /// Gets the current local DateTimeOffset
        /// </summary>
        public static DateTimeOffset NowOffset => _offsetHolder ?? DateTimeOffset.Now;

        /// <summary>
        /// Allow overriding the time for testing
        /// </summary>
        /// <param name="fakeDateTimeOffset"></param>
        public static void SetNowOffset(DateTimeOffset fakeDateTimeOffset)
        {
            _offsetHolder = fakeDateTimeOffset;
        }

        public static void ResetOffsetNow()
        {
            _offsetHolder = null;
        }
    }
}

SystemTime Alternative

using System;
 
namespace SystemTime
{
    public class SystemTime
    {
        protected static Func<DateTime> localNow = () => DateTime.Now;
        public static DateTime Now
        {
            get => localNow();
            protected set { localNow = () => value; }
        }
    }
}
    

using System;
using System.Collections.Generic;
using System.Text;
 
namespace SystemTime.Testing
{
    public class TestingSystemTime : SystemTime
    {
        public static DateTime StartForTest()
        {
            Now = DateTime.Now;
            return Now;
        }

        public static DateTime Reset()
        {
            localNow = () => DateTime.Now;
            return localNow();
        }

        public static DateTime AddMinutes(int mins)
        {
            Now = Now.AddMinutes(mins);
            return Now;
        }

        public static DateTime AddHours(int hours)
        {
            Now = Now.AddHours(hours);
            return Now;
        }
    }
}