[ntldr1962]

In internet you can find many times this formula:

=(((A1/60)/60)/24)
or
=(((A1/60)/60)/24)+DATE(1970,1,1)
or
=(((A1/60)/60)/24)+DATE(1970,1,1)+(-5/24)
or
=(A1/86400)+25569+(-5/24)

If somebody have the same problem as me, is necessary to know that this number in the datacolumn is the Unix time is defined as the number of seconds since midnight (GMT time) on January 1, 1970 -- also known as the Unix epoch.

The correct formula to calculate it is:

=(Y2-$Y$1)*86400 -> in excel 2010 where Y2= cell containing the most recent date
Y1= cell with the value 01/01/1970 00:00:00

Remember that is important to use the same format on date cells and in the result cell to have the format of number

I leave all this here in order to help to somebody who can need it.