Why though? What business does one employer have knowing what else you do in your life? Do you tell them what hobbies you’re into or what tv show you watched last?
Legally speaking because it's in your contract. Ethically it's because when you agree to a full time position you are agreeing to make your time available, with reasonable allowances. Again, it's completely fine to make a different arrangement if that's clear up front.
Unless you are somehow working them in totally offset hours, or you have _literally_ zero meetings and work in a completely asynchronous environment, then you need to disclose.
But that's not true for an overwhelmingly large number of people. You aren't just being paid for output, you are also being paid for time.
Legally I have had one employer contract call out that they expect you to work for certain hours, but that was a Canadian startup; none of the American companies have had that I remember.
Ethically though, I don't feel this way at all. They're paying for my output not my time. I don't get paid overtime when I work extra nor do I get credit for working on the weekend.
I don't think I'll convince you but I guess I would say you don't owe your employer your life. They'll cut you off when it's to their benefit and it's best you do what's best for you.
