How to set date and time with a given string??

 Hi
I have to set date and time. So I tried Date and TimeOfDay properties. But they are 'get' and not 'set' properties. There are so many threads on DateTime but I didn't come across what i wanted. Very likely i missed it.
There is
 _txtAddDate.Text  //contains user-input Date Ex: 01/01/2000
_txtAddTime.Text //contains user-imput Time. Ex: 5.40 pm
Suppose there is DateTime d. How can I set Date and Time of d?
 
Any replies appreciated.
 
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