Vilhelm,
in your formula, is S2 a date? Or is the result of the formula a date
(in which case dates are in column G:G)?
OFFSET() will find reference a cell as an offset from its position, not
its value. VLOOKUP() finds a value, not a position. MATCH() will find a
relative position within an array. Thus,
MATCH(VLOOKUP(...)-1, ...) would find the location of the previous
value in the array.
INDEX() will find the nth element in an array. The combination
INDEX(..., MATCH(...)) is often used as a substitute of VLOOKUP when
there is more complexity in the required calculations than VLOOKUP()
can handle.

Write back if you need something more specific or get stuck.

HTH
Kostis Vezerides