Duration field that accepts a number of weeks and days

I have a duration field that looks like this. The user can set a time in HH:mm

However, in some cases the user might have to set a duration in weeks/days. This is the current design I have:

The placeholder "ww-d" disappears when the user starts typing and it's replace with "__-_".

Is there a more intuitive and user friendly way to display a duration field that accepts a number of weeks/days? thank you.