Question

I have a reminder table in my database that has 2 columns to store the ActionDay and ReminderDay. These days are both stored as integers: 0 (sun) -> 6 (sat) to represent the days of the week.

When I load my page I’d like to show the next ActionDay date and along with the ReminderDay date.

I can work out the next ActionDay date using this:

public static class DateTimeExtensions
{
    public static DateTime Next(this DateTime date, DayOfWeek weekday)
    {
        return (from i in Enumerable.Range(0, 7)
                where date.AddDays(i).DayOfWeek == weekday
                select date.AddDays(i)).First();

    }
}

I’m having some trouble working out the previous date from the ActionDay.

For example, (using today’s date of 26 Oct 2011):
If the ActionDay is ‘3’ and the ReminderDay is ‘2’ then I expect
‘Wed, 26 Oct’ for the ActionDay and ‘Tues, 25 Oct’ for the ReminderDay.

If the ActionDay is 0 and the ReminderDay is 6 I’d expect
‘Sun, 30 Oct’ and ‘Sat, 29 Oct’

Any suggestions on how I might be able to get this Reminder date? I’d also be interested to know if there are any good c# datetime extensions/snippets as it might be easier to plug in something that someone else has tested than writing my own here 🙂

Thanks,
Rich

Answer 1

You can calculate the difference between the ActionDay and ReminderDay and then call the AddDay() function on the DateTime representing the action day to get the DateTime for reminder day:

// here a stands for the number representing the action day, and r for the reminder day
// a and r are both integers from 0 to 6
int diff;
if (a >= r)
{
    diff = a - r;
}
else
{
    diff = a + 7 - r;
}

DateTime reminderDateTime = actionDateTime.AddDays(-1 * diff);