We use vega and it doesn't appear to be possible to format axis ticks (and other objects such as tooltips, text marks) as duration.
Consider a data column that contains the amount of seconds elapsed (an integer representing the duration in seconds), we would like to format the ticks as d.hh:mm:ss
But this doesn't appear possible.

The type is number and not Date, so we cannot use time formats.
Even if we convert our integer to a Date, it's still not right because durations shouldn't be formatted like dates.
At first glance, you might think that we could use the format '%d.%H:%M:%S'. However, this won't work if the duration is greater than 31 days, because %d represents the day of the month, whereas what we really want is to show the total number of elapsed days.

I think the right solution would be to have a native format for durations (integer as seconds).