Display minutes and seconds only.

Hi,

I am using report builder 3.

I want to use an expression to calulate the difference between two Datetime fields and display the result in minutes and seconds.

I am using the following expression.

=IIf(Fields!Initials.Value = Previous(Fields!Initials.Value),DateDiff("s",Previous(Fields!Datetime.Value),Fields!Datetime.Value),"")

The above calculates the seconds using "s"
"n" will calculate the minutes.

Am trying to use the Format option but can not get the syntax right.

Any help would be appreciated

Steve