Bill your clients based on times when a Terminal with their project was running (assuming that's the time you spent developing it).

billable_hours.rb

#!/usr/bin/env ruby

require 'terminal.com'
require 'date'

# Usage: ./billable-hours.rb 2015/9/1
invoice_from = Date.parse(ARGV.shift).to_time
# response = Terminal.terminal_usage_history("user_token", "access_token")
response = JSON.parse(DATA.read)

projects = response['usage'].values.reduce(Hash.new) do |projects, blob|
  blob['intervals'].each do |interval|
    start_timestamp, end_timestamp = interval.values_at('start', 'end')
    start_time, end_time = Time.at(start_timestamp.to_i), Time.at(end_timestamp.to_i)
    if start_time < invoice_from
      projects[blob['subdomain']] ||= Array.new
      projects[blob['subdomain']] << (start_time..end_time)
    end
  end
  projects
end

projects.each do |project, billable_intervals|
  dates = billable_intervals.group_by { |interval| interval.first.strftime('%d/%m') }
  dates.each do |date, billable_intervals|
    seconds_spent = billable_intervals.reduce(0) do |sum, interval|
      sum + (interval.end - interval.begin)
    end
    puts "#{date}: You spent #{(seconds_spent / 3600).round(2)} hours on #{project}"
  end
end

# projects = response['usage'].values.reduce(Hash.new) do |projects, blob|
#   projects[blob['subdomain']] ||= Array.new
#   blob['intervals'].each do |interval|
#     start_time, end_time = interval.values_at('start', 'end')
#     difference = Time.at(end_time.to_i) - Time.at(start_time.to_i)
#     projects[blob['subdomain']] << difference
#   end
#   projects
# end

# sort_by { |blob| blob['subdomain'] }#.keys

__END__
{
    "usage": {
        "00000000-0000-0000-0000-000000000000": {
            "subdomain": "test1",
            "container_key": "00000000-0000-0000-0000-000000000000",
            "intervals": []
        },
        "00000000-0000-0000-0000-000000000001": {
            "subdomain": "test2",
            "container_key": "00000000-0000-0000-0000-000000000001",
            "intervals": [
                {
                    "start": "1399075766",
                    "end": "1401230005",
                    "state": {
                        "cpus": 2,
                        "ram": 256,
                        "disk": 10317112,
                        "status": "running"
                    }
                }
            ]
        }
    }
}