Most of the replies here paint the discussion as you against your employer, represented by your manager. In my experience things are a bit more nuanced. Even when your manager trusts you and knows your worth, he often does not have the authority to give you the raise you both agree you deserve. In that context, taking a day off to interview with another company and not hiding it from your manager may actually give him useful arguments when he has to argue for a budget increase so he can give you that raise.