[R] Help for selecting dates

[Hans-Jürgen Auinger]

Hi people,

my problem is, that I have to get the average daily gain of goats through the first eight weeks and the following five month. 
I have the birth weight and about every four week a new weight. 
Now I like to know, how I can choose the weight next to the age of eight week of the goats, so that IŽm able to get the average daily gain in the time before that date and after that date. Maybe you know where to look for an answere to my question.

Thanks for helping and regards

Hans-Jürgen Auinger