I'll try to explain it better here. The whole reason month=2 works is because we have a starting point for months. The starting point is inside of the year. So, every second month means you aways return Jan, Mar, etc. The same can be said for days, hours, minutes, seconds, as they all have larger intervals from which to start. However, week doesn't have a larger interval. A month doesn't always start at the beginning of the week, and the same can be said for a year (or even the start of the unix epoch). Because of this, there is no way to know when every second week is or every third week is