encode-universal-time performance

[sshteingold@cctrading.com]

     The major bottleneck of my code is computing the number of days 
     between 2 dates. Now I convert both dates to the universal time 
     (bignum of secs since the epoch) and take the difference divided by 
     24*60*60. Is there a faster way to do that? profiling shows that I 
     spend all the time encoding universal time, and I wonder if there is a 
     way to avoid that. Apparently the best thing I will be able to do will 
     be using FFI and writing the thing in C. Is there a better way?