[R] select previous date

[alessandro carletti]

Hi everybody,
could anyone help me in finding a way for selecting
from a dataframe each row that comes before another
that matches a condition?

EXAMPLE:
df
raw.number  name     date      temp
1            aaa   2001-04-15   15
2            aaa   2001-01-15   12
3            aaa   2001-03-15   13
...
i-1          bbb   2002-04-15   15 
i            bbb   2002-03-15   14

the condition is: 
df$temp==15
matching raws are 1 and i-1:
I need something to select (only) rows where date=one
month before the matching raws, so raws 3 and i.
(the variable name has more than one level)
Thanks