// String to DateTime
 String MyString;
 MyString = "1999-09-01 21:34 PM";
 //MyString = "1999-09-01 21:34 p.m.";  //Depends on your regional settings

 DateTime MyDateTime;
 MyDateTime = new DateTime();
 MyDateTime = DateTime.ParseExact(MyString, "yyyy-MM-dd HH:mm tt",null);


 //DateTime to String
 MyDateTime = new DateTime(1999, 09, 01, 21, 34, 00);
 String MyString;
 MyString = MyDateTime.ToString("yyyy-MM-dd HH:mm tt");

/*
Format String For Dates

Your format string is your most important key. In most of my projects, I make it a constant and then refer to the constant value in my code.

The following is the most commonly used format characters:

d - Numeric day of the month without a leading zero.
dd - Numeric day of the month with a leading zero.
ddd - Abbreviated name of the day of the week.
dddd - Full name of the day of the week.

f,ff,fff,ffff,fffff,ffffff,fffffff - 
	Fraction of a second. The more Fs the higher the precision.

h - 12 Hour clock, no leading zero.
hh - 12 Hour clock with leading zero.
H - 24 Hour clock, no leading zero.
HH - 24 Hour clock with leading zero.

m - Minutes with no leading zero.
mm - Minutes with leading zero.

M - Numeric month with no leading zero.
MM - Numeric month with a leading zero.
MMM - Abbreviated name of month.
MMMM - Full month name.

s - Seconds with no leading zero.
ss - Seconds with leading zero.

t - AM/PM but only the first letter. 
tt - AM/PM ( a.m. / p.m.)

y - Year with out century and leading zero.
yy - Year with out century, with leading zero.
yyyy - Year with century.

zz - Time zone off set with +/-.
*/

// String to DateTime ========================================================

 // String to DateTime
 String MyString;
 MyString = "1999-09-01 21:34 PM";
 //MyString = "1999-09-01 21:34 p.m.";  //Depends on your regional settings

 DateTime MyDateTime;
 MyDateTime = new DateTime();
 MyDateTime = DateTime.ParseExact(MyString, "yyyy-MM-dd HH:mm tt",null);


// DateTime to String ========================================================

 //DateTime to String
 MyDateTime = new DateTime(1999, 09, 01, 21, 34, 00);
 String MyString;
 MyString = MyDateTime.ToString("yyyy-MM-dd HH:mm tt");

/*
Format String For Dates

Your format string is your most important key. In most of my projects, I make it a constant and then refer to the constant value in my code.

The following is the most commonly used format characters:

d - Numeric day of the month without a leading zero.
dd - Numeric day of the month with a leading zero.
ddd - Abbreviated name of the day of the week.
dddd - Full name of the day of the week.

f,ff,fff,ffff,fffff,ffffff,fffffff - 
	Fraction of a second. The more Fs the higher the precision.

h - 12 Hour clock, no leading zero.
hh - 12 Hour clock with leading zero.
H - 24 Hour clock, no leading zero.
HH - 24 Hour clock with leading zero.

m - Minutes with no leading zero.
mm - Minutes with leading zero.

M - Numeric month with no leading zero.
MM - Numeric month with a leading zero.
MMM - Abbreviated name of month.
MMMM - Full month name.

s - Seconds with no leading zero.
ss - Seconds with leading zero.

t - AM/PM but only the first letter. 
tt - AM/PM ( a.m. / p.m.)

y - Year with out century and leading zero.
yy - Year with out century, with leading zero.
yyyy - Year with century.

zz - Time zone off set with +/-.
*/

http://www.codeproject.com/KB/cs/String2DateTime.aspx?display=Print

Easy String to DateTime, DateTime to String and Formatting

date

C#