DATEDIFF("d", PREVIOUS({Date}), {date}) // {date} is your date field

This will give you the difference between the date and the one above it.  You have to put the formula in the details section.

Create a running total:

RTotal0 -
    Count the date
    Evaluate for every record
    Reset for each group

Create a few formulas:

In Details -
RECORD2 - Runs datediff only on the second record.  (Can be suppressed)
    if {#RTotal0} = 2 then {@DateDiff}

In the group header - 
VAR_RESET - Resets the Variable to 0 for each group. (Can be Suppressed)
    shared numbervar X := 0;

In the Group Footer -
VAR_DEC - Declares the Variable and returns the value for the second record
    shared numbervar X ;
    if {@RECORD2} > 0  then X := {@RECORD2};
    X

If this doesn't work, let me know ( it worked for me)