typedef union                          // union used for writing to TL0 & TH0
    {
        struct
        {
            unsigned char hi;
            unsigned char lo
        } b;
        unsigned int w;
    }udblbyte;

...
      TR0 = 0;                         // stop Timer0
      time.b.lo = TL0;                 // read lo byte first
      time.b.hi = TH0;                 // read hi byte second
      time.w = time.w - offset;        // calculate new time 
      TL0 = time.b.lo;                 // write lo byte first
      TH0 = time.b.hi;                 // write hi byte second
      TR0 = 1;                         // start Timer0