First And Last Criteria When Adding The Operation

I'm very confused with this "first" and "last" criteria

I can easily understand for "max" "min" "sum" etc ... but not the "first" and "last"

I import monthly data for the year 2013 and for each month I import, it will have a record showing 201301 (for january 2013), 201302 (for february 2013) etc... until today which is 201312

I import them in order that is: 201301 first till the last one which is 201312

Now is the Criteria "first" give me the date at which it is First entered in the table or just the first that he see in the table? ( can putting them in order either a->z or z<-a affect this criteria?) or what if I put import them not in order but randomly like?

When I used last it didn't give me 201312 but 201311...