How To Notify Users After 30 days of registration

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C#

Good Day all,

Please some one help me out here.

I have a database column for datecreated. I have written simple script to get the total number of days, that is comparing the datecreated to datetime.today, if the difference is >= 30 then send a mail to the user

The script is:

string datecreated = Convert.ToString(Session["datecreated"]);
            DateTime dt3 = DateTime.Parse(datecreated);
            DateTime dt4 = DateTime.Today;
            TimeSpan tp = dt4.Subtract(dt3);
            int bs = tp.Days;

The script gives the total days.
How can the server automatically send email to the user if tp.days > = 30?
Especially when the user is not logged in.

Thanks