Keep a copy in your ~/bin and chmod to 755. Now, you can compute nicely rounded hours at will from the command line.

hours.rb

#!/usr/bin/env ruby
##
# Convert hours to invoice time, which is rounded to 6 minute increments.
time_value = ARGV.last.to_s.strip
if time_value =~ /\A[0-9]*:[0-9]*\Z/
  time_elements = time_value.split(':')
  minutes = time_elements.first.to_i * 60.0 + 1.0 * time_elements.last.to_f
elsif time_value.to_f > 0.0
  minutes = time_value.to_f * 60.0
else
  STDERR.puts "Usage: hours 2:50 or hours 2.534 or hours 2"
  exit 1;
end

hours = (minutes / 60.0).round(1)
puts hours