Set an optional field to a fixed value

I've got a edit form in which the user can change a number of properties from an item. In certain conditions, there is an optional time field which the user can set. But it cannot be set to an actual value in the UI, the user can only specify that it must be set to 'Now' (a backend then later in the process will determine what 'Now' actually is with margins and so).

I am looking for a good design for this. Of course I can use a checkbox with "Use current time" or something, but that looks weird in this case.

What I've come up with is this: (real form and design is very different and specific, mockup just to show the idea): The optional field here is 'Due time'. When it's present and has no value, the 'Due time' header is shown with a grey 'Unknown' value below. A 'Now' button can be pressed and that changes the value to 'Now' after which the button disappears. User can Update the rest of the item or Cancel it by using the form buttons. We've got a Now/Clock icon and button in some other places in our app, so that might be familiar to the user.

What do you guys think? Is this a weird interaction? If don't think I see this much so I am doubting.