title: Excel now() shouldn't update existing timestamp
body: <p>I have an excel formula <code>=IF(NOT(ISBLANK(A2)), TEXT(NOW(), "mm/dd/yyyy HH:ss"),"")</code></p>
  
  <p>My Problem is: Of course, everytime I open this sheet it updates the cell value associated with most recent timestamp. I do not want this timestamp to change if it already existed.</p>
  
  <p>What I want is somehow: if A2 is not blank and wasn't updated recently then dont update the timestamp else update.</p>
  
  <p>I am looking for an inline function if possible.</p>
  
  <p>Thanks</p>