Forecast Date - MRP

Hello Gurus,

I have a simple question regarding Forecast Date. 

My client is giving me a Forecast Date, which is the date that they will begin building the item or buying the item.

For example:

13-January-2013, Quantiy 20.

Is the Forecast Date the date in which you should have finishing building the Assembly (so you have a quantity in inventory on-hand of 20), or the date you should begin building it?

As it make takes up to 1 month to build it.

One thing I know for sure is that I must have all the buy items bought for this date of 13-January-2014, so that I can start building my discrete Job.