Cron job run every three (or n) days?

[leafduan]

I want to run a job every three or five (or n) days from now, and I wrote the expression '0 8 /5 *'.

If now is '2018/05/17',

and I get the occurrences:

0 8 */5 * *
2018/5/21 8:00:00
2018/5/26 8:00:00
2018/5/31 8:00:00
2018/6/1 8:00:00
2018/6/6 8:00:00
2018/6/11 8:00:00
2018/6/16 8:00:00
2018/6/21 8:00:00
2018/6/26 8:00:00
2018/7/1 8:00:00
2018/7/6 8:00:00
2018/7/11 8:00:00
2018/7/16 8:00:00
2018/7/21 8:00:00
2018/7/26 8:00:00
2018/7/31 8:00:00
2018/8/1 8:00:00
2018/8/6 8:00:00

but, which I expected are:

0 8 */5 * *
2018/5/22 8:00:00     -- wrong
2018/5/27 8:00:00     -- wrong
2018/6/1 8:00:00
2018/6/6 8:00:00
2018/6/11 8:00:00
2018/6/16 8:00:00
2018/6/21 8:00:00
2018/6/26 8:00:00
2018/7/1 8:00:00
2018/7/6 8:00:00
2018/7/11 8:00:00
2018/7/16 8:00:00
2018/7/21 8:00:00
2018/7/26 8:00:00
2018/7/31 8:00:00
2018/8/5 8:00:00      -- wrong
2018/8/10 8:00:00     -- wrong

Do the cron expression that I wrote is wrong? Or the Cronos don't support this situation?

Any one help me?

Thanks, Leaf

[odinserj]

Cron expressions are never calculated based on the current time, so May 21, 2018 date will always match the 0 8 */5 * * expression, regardless you are asking to get ones since May 1, May 3, May 13, etc. You can consider Cron expressions as regular expressions, but for date/time values: their output is stable.

Simply speaking, every cron parsing algorithm looks like the following loop:

var data = startDate;
while (date < endDate)
{
    if (IsMatch(date, cronExpression))
    {
        yield return date;
    }

    date = date.AddSeconds(1); // Or minutes
}

And if you need to get occurrences every 3 days or so, the best term is to make your own loop and use the DateTime.AddDays method instead.

[leafduan]

Hi odinserj,

Thank you for your comment.

And I catch the same idea with you, override the get occurrences loop, using DateTime.AddDays instead.