title: I want to add 14 days / 30 days from created date in firestore database
body: <p>How can I add 14 days / 30 days to createdAt firestore date/time field, I will create another field called expiresIn to store future date after adding 14 days / 30 days..</p>
  <p>I am using <code>const timeStamp = admin.firestore.FieldValue.serverTimestamp();</code> to create createdAt using cloud functions onCreate method.. then will run schedule job to subtract remaining days and finally mark as Expired (or False).. How can add days to existing createdAt field ??</p>