interval.range()

interval.range(start, stop[, step])

Returns every an array of dates representing every interval boundary after or equal to start (inclusive) and before stop (exclusive). If step is specified, then every stepth boundary will be returned; for example, for the timeDay interval a step of 2 will return every other day. If step is not an integer, it is floored.

The first date in the returned array is the earliest boundary after or equal to start; subsequent dates are offset by step intervals and floored. Thus, two overlapping ranges may be consistent. For example, this range contains odd days:

d3.timeDay.range(new Date(2015, 0, 1), new Date(2015, 0, 7), 2);
// [Thu Jan 01 2015 00:00:00 GMT-0800 (PST),
//  Sat Jan 03 2015 00:00:00 GMT-0800 (PST),
//  Mon Jan 05 2015 00:00:00 GMT-0800 (PST)]

While this contains even days:

d3.timeDay.range(new Date(2015, 0, 2), new Date(2015, 0, 8), 2);
// [Fri Jan 02 2015 00:00:00 GMT-0800 (PST),
//  Sun Jan 04 2015 00:00:00 GMT-0800 (PST),
//  Tue Jan 06 2015 00:00:00 GMT-0800 (PST)]

To make ranges consistent when a step is specified, use interval.every instead.