law: Indian Contract Act, 1872 
  35. When contracts become void, which
  
  are contingent on happening of specified event within fixed time
  Contingent contracts to do or not to do anything, if a specified
  
  uncertain event happens within a fixed time, become void, if, at the expiration
  
  of the time fixed, such event has not happened, or if , before the time fixed,
  
  such event becomes impossible.
  When contracts may be enforced, which are contingent on
  
  specified event not happening within fixed time : Contingent contract to do or
  
  not to do anything, if a specified uncertain event does not happen within a
  
  fixed time, may be enforced by law when the time fixed has expired and such
  
  event has not happened, or before the time fixed has expired, if it become
  
  certain that such event will not happen.
   Illustrations 
  (a) A promises to pay B a sum of money if a certain ship returns
  
  within the year. The contract may be enforced if the ship returns within the
  
  year; and becomes void if the ship is burnt within the year.
  (b) A promises to pay B a sum of money if a certain ship does
  
  not return within a year. The contract may be enforced if the ship does not
  
  return within a year, or is burnt within the year.