Does anyone have any advice or a good tutorial for working with timestamps, and user configurable timezones within Drupal 6?

I have analytics data being recorded, each event with a timestamp generated via time(), so it's a standard Unix timestamp.

Previously, I think I had the analytics being reported correctly for each user in their local timezone via:

$timeStr = format_date($action->timestamp, 'custom', $customTimeFormatStr, variable_get('date_default_timezone', 0));

But when I added analytics exporting, with the user being able to specify a start and end date/time string, I began to suspect that the above timezone handling is incorrect.

Given: 
$start = '7 am';
$timecode = strtotime( $start );
$start = format_date( $timecode, 'custom', 'h:i:s a, d M Y', variable_get('date_default_timezone', 0));
 
I get $start is "11:00:00 pm, 10 Mar 2011' <- wrong time, correct date

However, changing the format_date() call to be like this:

$start = format_date( $timecode, 'custom', 'h:i:s a, d M Y', 0);   <- changed variable_get() to 0

Gives me "07:00:00 am, 11 Mar 2011"  <- correct time, but one day in the future

Leaving the timezone parameter off, letting it's default NULL value be used like this:

$start = format_date( $timecode, 'custom', 'h:i:s a, d M Y');

Gives me "12:00:00 am, 11 Mar 2011"  <- wrong time plus one day in the future

I guess I could just use the second form, with the timezone parameter set to zero, and recommend that users include the date, but it sure is convenient to just say "12:00:00 am" and "12:59:59 pm" to capture the analytics thus far for the day.

Perhaps my date time configuration for the site is incorrect? At admin/settings/date-time I have the time zone set correctly, and user configurable timezones are enabled. BTW, the users of the site are international, with people on both east & west coast US, London, Israel, and India. So the timezone handling is important. 
 
Sincerely,
-Blake