What's the logic behind the unreachable dates?

Hi! I am trying to familiarize myself with the scenarios in which the GetNextOccurrence can return a DateTime? with null value, and I have a hard time understanding the logic behind the behavior. It seems that calculating the next occurrence for dates after the year 2100 is unsuccessful about half of the time. Here is a minimal example:

string expression = "0 0 15 6 *"; // June, 15
var cronExpression = CronExpression.Parse(expression);
Console.WriteLine($"Expression: {cronExpression}");
for (int i = 1; i <= 24; i++)
{
    var fromUtc = new DateTime(2150, 1, 1, 0, 0, 0, DateTimeKind.Utc).AddMonths(i);
    var nextUtc = cronExpression.GetNextOccurrence(fromUtc);
    Console.WriteLine($"Date: {fromUtc}, NextOccurrence: {(nextUtc.HasValue ? nextUtc.Value.ToString() : "-")}");
}

Output:

Expression: 0 0 0 15 6 *
Date: 02/01/2150 00:00:00, NextOccurrence: 06/15/2150 00:00:00
Date: 03/01/2150 00:00:00, NextOccurrence: 06/15/2150 00:00:00
Date: 04/01/2150 00:00:00, NextOccurrence: 06/15/2150 00:00:00
Date: 05/01/2150 00:00:00, NextOccurrence: 06/15/2150 00:00:00
Date: 06/01/2150 00:00:00, NextOccurrence: 06/15/2150 00:00:00
Date: 07/01/2150 00:00:00, NextOccurrence: -
Date: 08/01/2150 00:00:00, NextOccurrence: -
Date: 09/01/2150 00:00:00, NextOccurrence: -
Date: 10/01/2150 00:00:00, NextOccurrence: -
Date: 11/01/2150 00:00:00, NextOccurrence: -
Date: 12/01/2150 00:00:00, NextOccurrence: -
Date: 01/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 02/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 03/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 04/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 05/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 06/01/2151 00:00:00, NextOccurrence: 06/15/2151 00:00:00
Date: 07/01/2151 00:00:00, NextOccurrence: -
Date: 08/01/2151 00:00:00, NextOccurrence: -
Date: 09/01/2151 00:00:00, NextOccurrence: -
Date: 10/01/2151 00:00:00, NextOccurrence: -
Date: 11/01/2151 00:00:00, NextOccurrence: -
Date: 12/01/2151 00:00:00, NextOccurrence: -
Date: 01/01/2152 00:00:00, NextOccurrence: 06/15/2152 00:00:00

Live demo.

Could you explain why the calculation fails when the month+day of the starting date is larger than the month+day in the Cron expression?

Thanks!

[lonix1]

In 78 years when my system fails, I’m coming back here to complain. I’ll be angry, and there will probably be swearing… 😉 😛

[odinserj]

Cronos wasn’t planned to calculate occurrences after 2099 year, there’s a hardcoded values, please see the following snippet. Such an upper bound is required, because it’s not possible to know that some cron expression leads to an unreachable date in upfront, and we need to avoid the infinite loop in this case.

https://github.com/HangfireIO/Cronos/blob/b35c34692f74cf59630be8184d12742306ab2b32/src/Cronos/CronExpression.cs#L42

I think there should be ArgumentOutOfRangeException thrown when start date’s year is more than this hardcoded value.