Mea culpa. Choosing this name was really unfortunate and lead to many incomprehension.
Thus, I have decided to rename this parameter the period reference. It's default value is 2001-01-01 (i.e. Monday, January 1st, 2001), and unless you know why you need to change this value, I would strongly suggest you did not.
Let's start with a practical example. Let's assume that, as a retailer, you need monthly sales forecasts in order to make your monthly sales replenishment orders. Yet, your business months are starting the 15th of each month. All your suppliers are expecting your orders to be passed the 15th, and for years, all your monthly sales analysis have been starting the 15th.
In such a situation, it would be a real pain if Lokad was arbitrarily deciding that a month had to be starting the 1st. In order to avoid such a pitfall, Lokad provides an additional setting for the forecasting task definition that lets you adjust when you want the period to start: it's the period reference.
If you want your monthly forecasts to start the 15th of each month, then you can use 2001-01-15 as your period reference (or 1999-02-15 or 2017-11-15, the result would be the same). This date is used as a reference to infer all the other period's starting dates.
- If the period reference is set to 2001-01-15 for a yearly forecast, then all years start January 15h (instead of January 1st).
- If the period reference is set to 2001-01-15 for a weekly, then all weeks start on Tuesdays (because 2001-01-15 was a Tuesday).
In summary: The period reference, previously named "period start", is a date (past or future, it does not matter) used by Lokad to adjust the period boundaries both for historical data aggregation but also for the forecasts themselves. In particular, it has nothing to do with the starting date of the forecasts.