I make a half day the minimal billable unit for support of old projects. This helps control the problem of odd hours popping up here and there.
For companies that send you around the place internationally, I charge full days for international travel time - giving them the choice of sending you Business or Economy (and the corresponding amount of work that would be possible to undertake in flight).
EDIT: Re the last point - If they are paying by the hour/day/week, they are buying my time irrespective of whether I am able to work or not. Eg If they put me on economy, or put me through a day of meetings - they are going to be charged the day.
I'm not 100% clear on your last point, are you saying that if the send you Business-class you'll be able to get some amount of work done on the plane that you won't if Economy? I'm not arguing here, just trying to understand.
Can we speak a little more as to how you indicate that a half day is the minimal billable unit? My clients are happy to pay my high rate but I can't imagine ever suggesting to them that anything but an hour would be the minimal unit at this point.
I simply tell them billable time for support on old projects is rounded up to 4 hour blocks - this is with or without a retainer / support agreement. This covers switching time (pulling up virtual machines, getting your notes out etc, finding a Windows XP laptop etc), and scheduling it into current work schedule. I work with the client to ensure we gather work into 4 hour blocks where possible (eg. get them to build a todo list in Jira or equivalent). The idea is to avoid death by 1000 cuts - ie dribs and drabs of 1 hour jobs that they call thru at random.
Did you know that temp agencies for office staff -- receptionists, AAs, clerks, WP -- have a four-hour minimum too? WP can get as high at $25/hr for third-shift legal.
For companies that send you around the place internationally, I charge full days for international travel time - giving them the choice of sending you Business or Economy (and the corresponding amount of work that would be possible to undertake in flight).
EDIT: Re the last point - If they are paying by the hour/day/week, they are buying my time irrespective of whether I am able to work or not. Eg If they put me on economy, or put me through a day of meetings - they are going to be charged the day.