How to set delegation for vacation period?

Hi Expert, 

When an approver(e.g:David in the screenshot below) is on vacation,
he wants to delegate his approval requests that come from different ItemType such as ECO or DCO to someone else(Avilash).
So, I tried to set delegate person (Avilash) in Member tab of Identity (David's identity) with 'From Date' and 'To Date' information.
However, once this configuration was done, when new ECO was created with Team 'Product' as below, approval request didn't go to delegate person (Avilash) but original appprover (David).

Would you please help me how to do this delegation?

Thank you so much.