Use AddSeconds

using System;

public static class Gigasecond
{
    public static DateTime Add(DateTime birthDate)
    {
        return birthDate.AddSeconds(1_000_000_000);
    }
}

The DateTime class has an AddSeconds() method that takes the number of seconds to add to the DateTime instance. We then simply pass in the gigasecond value and a new DateTime instance is returned with the amount of seconds specified added to it.

Shortening

There are two things we can do to further shorten this method:

1. Remove the curly braces by converting to an expression-bodied method
2. Replace 1_000_000_000 with 1e9, which is the same number but in scientific notation

Using this, we end up with:

public static DateTime Add(DateTime birthDate) => birthDate.AddSeconds(1e9);