I tried to group the calendar events by the start field grouped by day, week and month.

Here is the code I tried

<record id="meeting_group" model="ir.ui.view">
           <field name="name">meeting.group</field>
           <field name="model">calendar.event</field>
           <field name="inherit_id" ref="calendar.view_calendar_event_search"/>
           <field name="arch" type="xml">
               <xpath expr="//search" position="inside">
                   <filter string="day" domain="[]" context="{'group_by':'start:day'}"/>
                   <filter string="Week" domain="[]" context="{'group_by':'start:week'}"/>
                   <filter string="Month" domain="[]" context="{'group_by':'start:month'}"/>
               </xpath>
           </field>
       </record>

This is expected to work fine but but the grouping is not done properly. I get incorrect dates in the group. For example in the group 26/9/2018 there are other date records like 27/9/2018. how to correct?