dateTime.ToString() / dateTime.AddDays()

// Converts a DateTime to a string using the specified format.
string s = dateTime.ToString(string format);

// Adds the specified number of days (use a negative value to subtract).
DateTime newDateTime = dateTime.AddDays(double value);

// Other add methods follow the same pattern.
DateTime newDateTime = dateTime.AddHours(double value);
DateTime newDateTime = dateTime.AddMinutes(double value);
DateTime newDateTime = dateTime.AddMonths(int months);
DateTime newDateTime = dateTime.AddYears(int years);

using System;

DateTime now = new DateTime(2024, 1, 15, 14, 30, 45);

// Use ToString() with a format string to convert to a string.
Console.WriteLine(now.ToString("yyyy/MM/dd"));         // 2024/01/15
Console.WriteLine(now.ToString("yyyy/M/d"));           // 2024/1/15
Console.WriteLine(now.ToString("HH:mm:ss"));            // 14:30:45
Console.WriteLine(now.ToString("yyyy/MM/dd HH:mm"));   // 2024/01/15 14:30
Console.WriteLine(now.ToString("yyyyMMdd"));            // 20240115

// You can also use format specifiers in string interpolation.
Console.WriteLine($"{now:yyyy/M/d (ddd)}");            // 2024/1/15 (Mon)

// Use AddDays() to add days.
DateTime dayAfterTomorrow = now.AddDays(2);
Console.WriteLine(dayAfterTomorrow.ToString("yyyy/MM/dd")); // 2024/01/17

// Pass a negative value to subtract days.
DateTime yesterday = now.AddDays(-1);
Console.WriteLine(yesterday.ToString("yyyy/MM/dd")); // 2024/01/14

// You can also add months, years, and hours.
Console.WriteLine(now.AddMonths(3).ToString("yyyy/MM/dd"));  // 2024/04/15
Console.WriteLine(now.AddYears(1).ToString("yyyy/MM/dd"));   // 2025/01/15
Console.WriteLine(now.AddHours(8).ToString("HH:mm"));        // 22:30

Methods like AddDays() do not modify the original DateTime value. They return a new DateTime as the result, so always assign the return value to a variable.

In format strings, the basic specifiers are yyyy (4-digit year), MM (2-digit month), dd (2-digit day), and HH (24-hour hour). Note that M (uppercase) means month while m (lowercase) means minutes — mixing up the case will produce unexpected output.

To calculate the difference between two date/time values, see 'TimeSpan / Date Difference'.