We have 2-objects called A,B and A is the parent and B is child. We need to update X-field value(Date data type) on B object with A object's Y-field value(Date) whenever A-object's STATUS changes to certain values(pick list) . Can we achieve this by using formula field. I'm bit confuse to choose correct solution.Please help me here.Thanks.
If I understand you correctly, you have a Date value field on both the parent and child, and you want to update the value on the child if and only if another field on the parent is changed. You do not want the fields to be kept in sync otherwise.
If this is correct, you cannot use a formula field, because formula fields are always "live" connections. They do not store values; they are calculated at the time of viewing the record based on their formulae.
If your records have a master-detail relationship, you can use a Process on the parent object to update field values on all of its children based on criteria that you specify. Your process can run whenever the parent object is modified and perform Date updates only when your picklist field contains specific values.