Timer

Error Im getting.. in 'sleep' : time interval must be positive <Argument Error>

def time_block
   start_time = Time.now
   yield
   data = Time.now - start_time
   #return Time.now - start_time
   return data.abs
end

def repeat_every(seconds)
   while true do
     # begin
     time = sleep( seconds - time_block { yield } ) # again ignoring time > seconds
     puts time
     #rescue
     #puts "Time interval was not positive"
     #end #begin
   end
end

repeat_every(8.0) do
   checkMail
end