C#:
Calculating a date in the future or past

How to:

Calculating future dates:

using System;

class DateExample
{
    static void Main()
    {
        DateTime currentDate = DateTime.Now;
        TimeSpan oneWeek = TimeSpan.FromDays(7);
        
        DateTime nextWeek = currentDate + oneWeek;
        Console.WriteLine($"One week from now: {nextWeek}");
    }
}

Output:

One week from now: <date a week from the current date>

Calculating past dates:

using System;

class DateExample
{
    static void Main()
    {
        DateTime currentDate = DateTime.Now;
        TimeSpan tenDaysAgo = TimeSpan.FromDays(-10);
        
        DateTime pastDate = currentDate + tenDaysAgo;
        Console.WriteLine($"Ten days ago was: {pastDate}");
    }
}

Output:

Ten days ago was: <date ten days before the current date>

Deep Dive

In C#, DateTime and TimeSpan are the bread and butter for date and time operations. DateTime represents an instant in time, typically expressed as a date and time of day. TimeSpan represents a time interval.

Historically, date and time calculations were prone to errors due to manual handling of days, months, and leap years. DateTime abstracts these complexities, letting the framework handle the tricky bits.

Alternatives to DateTime and TimeSpan in .NET include DateTimeOffset, which includes a time zone offset, making it better for applications that work across time zones. Another alternative is Noda Time, a library by Jon Skeet designed for more complex date and time handling, like differing calendars.

Implementation-wise, when you add a TimeSpan to a DateTime, under the hood, it’s manipulating ticks, the fundamental unit of time in .NET (1 tick = 100 nanoseconds). For past dates, a negative TimeSpan does the trick.

See Also

• .NET API documentation for DateTime
• Introduction to TimeSpan
• Microsoft’s best practices for DateTime and DateTimeOffset
• Noda Time documentation: https://nodatime.org