awk -F: '{
  FILENAME=="in_file" {in_total += $1 * 86400 + $2 * 3600 + $3 * 60 + $4}
  FILENAME=="out_file" {out_total += $1 * 86400 + $2 * 3600 + $3 * 60 + $4 }
  END { printf("%f\n", in_total - out_total) }
}' in_file out_file