JavaScript date : How to get days between two dates?

var daysInbetween = (d2.getTime() - d1.getTime())/(1000 * 3600 * 24);

SPQuery: How to filter based on modified date?

<Query>
    <Where>
        <Leq>
            <FieldRef Name="DateColumn" />
            <Value Type="DateTime">
                <Today OffsetDays="-4" /> // Modified in last 4 days
            </Value>
        </Leq> 
    </Where>
</Query>