How to combine two strings (date and time) to a single DateTime

Your issue is with your hour specifier; you want h (The hour, using a 12-hour clock from 1 to 12), not HH (The hour, using a 24-hour clock from 00 to 23).

Try like this;

string one = "13/02/09";
string two = "2:35:10 PM";

DateTime dt = Convert.ToDateTime(one + " " + two);
DateTime dt1 = DateTime.ParseExact(one + " " + two, "dd/MM/yy h:mm:ss tt", CultureInfo.InvariantCulture);

Console.WriteLine(dt1);

Here is a DEMO.

HH using a 24-hour clock from 00 to 23. For example; 1:45:30 AM -> 01 and 1:45:30 PM -> 13

h using a 12-hour clock from 1 to 12. For example; 1:45:30 AM -> 1 and 1:45:30 PM -> 1

Check out for more information Custom Date and Time Format Strings