There's no implicit conversion from Nullable<T> to T - you can do it explicitly via a cast, e.g. new DateTime((long) ticks), or use the Value property, e.g. new DateTime(ticks.Value).

As an alternative that I'd generally prefer now, you can use pattern matching from C# 7 to make it slightly simpler, extracting the non-null value in the same step where you check that it is non-null:

public static void Add(long? ticks)
{
    // This will match if ticks is non-null, and assign the value
    // into the newly-introduced variable "actualTicks"
    if (ticks is long actualTicks)
    {
        var dt = new DateTime(actualTicks);
        // Presumably use dt here
    }
}

You can implicitly widen from long to long?, but you can't implicitly narrow from long? to long.

You need to do this:

public static void Add(long? ticks)
{
    if (ticks != null)
    {
        new DateTime(ticks.Value);  // or (long)ticks
    }
}