How to respond an email for postdoc process

Congratulations on your new job!

They want to know at which day you can start the job, in order to include the starting date in your contract, which I presume ms. YYY and ZZZ are drawing up as we speak.

I would interpret this as: which date is it possible for you to come to the Netherlands to start the job.

But I think the best approach is to give a slightly more details on when you are available for what. You could say "I'm available to discuss any issues around setting up on the job on X and Y days, between Z and W hours. I'll be ready to move to the Netherlands whenever my permit is ready/any time after April 12/let me get back to you in a week or two with a precise date."