In .NET Framework you work with DataTime objects when you're creating such schedules and you have many options where you get to add seconds, minutes or even years and similarly you can subtract them from each other and so on.
I have written an article that explains this, and you can learn it there.
Understanding DateTime struct in .NET Framework (using C#)[
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Somehow, I do want to post the very basic code here, to make you understand it without having to go through that entire article (but do give it a read, it will explain the basics about DateTime object in the .NET Framework).
DateTime dateTime = DateTime.Now;
dateTime = dateTime.AddDays(1);
Same thing goes for subtract, you just pass a negative value to it.