title: Subtract time from date using format string
body: <p>If I have a date and time in a DateTime object, can I remove say 10 minutes, or 24 hours etc from the date and time using a format string?</p>
  
  <p>So if I have 1/1/1990 12:30:00pm and I wanted to remove 1 hour from it, can I use a format string?</p>
  
  <p><strong>edit</strong></p>
  
  <p>i need to store diary entries and the user can select a reminder type. so 1 hour before hand. so then i'd like to store the format string in a db that i can get and apply to a datetime to get the reminder date time</p>