How can this fail?

It looks like the interpreter thinks you're dealing with a literal empty string, not a zero (or other number). This is from a normal TCL command line, not a BigIP, but it's pretty easy to replicate:

 

 

 
 % set one [clock seconds] 
 1234571258 
  
 % set two "" 
 % expr { $one - $two } 
 can't use empty string as operand of "-" 
 % 
 % set two 0 
 0 
 % expr { $one - $two } 
 1234571258 
 % 
  
 

 

 

So this hints that you may need to look back at the code and make sure that $start_time is correctly set to something other than an empty string.

 

 

HTH,

 

-Matt