What does one question mark following a variable declaration mean?

This is a nullable type. Nullable types allow value types (e.g. ints and structures like DateTime) to contain null.

The ? is syntactic sugar for Nullable<DateTime> since it's used so often.

To call ToString():

if (timstamp.HasValue) {        // i.e. is not null
    return timestamp.Value.ToString();
}
else {
    return "<unknown>";   // Or do whatever else that makes sense in your context
}

? makes a value type (int, bool, DateTime, or any other struct or enum) nullable via the System.Nullable<T> type. DateTime? means that the variable is a System.Nullable<DateTime>. You can assign a DateTime or the value null to that variable. To check if the variable has a value, use the HasValue property and to get the actual value, use the Value property.