when i run code shows string was not recognized validdatetim

the code as follows;
 
DateTime dateofbirth = Convert.ToDateTime(txt_DOB.Text.ToString());
DateTime Weddingday = Convert.ToDateTime(txt_wedding.Text.ToString());
 
if(dateofbirth.Year.ToString()!="1900")
SendSMS(txt_DOB.Text.Trim());
else if(Weddingday.Year.ToString()!="1900")
SendSMS(txt_wedding.Text.Trim());
 

when i run this code error as follows;
 
string was not recognized a valid date time.
 
from my above wat is the problem help me.