If the date of khums has come what does it mean. Does it mean that the money has to go on that day? Or the calculation has been done and niyyah should be made by that day even if you actually give it at a later date.

Supposing If you have done the calculations and made the niyyah but you couldn't take out money from the bank so you don't have it in your hand at that time and the person who you have to handover the money to take it to the Marjaa will travel after few months. So anyways the money will stay with you either in the bank or in your hand.
So is this acceptable?

It means that you have to pay it on that day, and if you are not able to hand it over on that day, then you should seek permission to separate it from the remaining 80%, even by transferring it to another account, or to a sub account, so that you can use the rest of the money.