Date format problem

[Jovica]

Hi,

all my date entries:
31.12.2012.
31 12 2012
31.12.2012
31/12/2012
31-12-2012

can not be converted to dates. I am using "dd.mm.yyyy." as date format and never had any problems with it until I've tried to use that values in calculations (simple if...this date>that date).
I've realized that calculations are only possible with mm.dd.yyyy.format.

My question is, how can I continue to use dd.mm.yyyy. when entering dates in cells and in the same time retain their functionality?