Try to skip the date formatting. It should work. And if you have the computed field, set the field type datetimeDateTime.
Maulik.App.ContentSearch.ComputedIndexFields.DateForBoosting,Maulik.App.ContentSearch
<field fieldName="date" returnType="datetime">
Maulik.App.ContentSearch.ComputedIndexFields.DateForBoosting,Maulik.App.ContentSearch
</field>