Add SystemTime example

[aligneddev]

Add links and code example of a testing seam for DateTime.

I've called this systemTime in the past.
Here's a possible implementation
using System;

namespace Shared.Core.Implementations
{
///

/// Testable alternative to DateTime.Now or DateTimeOffset.Now
///

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;
 }
}

}

[aligneddev]

Here's another option:
public class SystemTime
{
protected static Func localNow = () => DateTime.Now;
public static DateTime Now
{
get => localNow();
protected set { localNow = () => value; }
}
}

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;
 }
}