Nothing takes the fizz out of feature like not shipping on time. The more the delay the more frustrating it gets for everyone - the team, the stakeholders and the customers. That doesn't mean, you deliver something haphazard just because you are nearing the deadline. The ideal is deliver what you promised and on time. So how do you go about ensuring that you deliver on time, or at the very least not frustrate the customers.
Break down the feature into smaller chunks or user stories or work items.
Put ETAs for each of the above items and call out owners.
Check periodically - daily stand-ups are very helpful. Keep track of ETAs and check in the mid point and 75% point. Say its a 5 day work item, then check at 2 day and 4-day mark with the owner if the item is going per schedule
Following the above steps, usually you get an idea of slippages. Once you spot a slippage, see if any remediation steps can be taken. If nothing can be done to stop the slippage and you are sure you are going to overshoot the ETA, first and foremost, inform the stakeholders. They might even find a way to put more resource on the problem and help you if it is a business critical feature. As for the customers keep them posted through forum posts or emails (if the count is small), better still you can put out the mocks to instill confidence that the feature is being worked on and call out the new ETA. Better still, you can call out the chunks that you will not be delivering in v1 on the time of earlier ETA, but you are still open to release as a private beta to your customers on demand. You can read more about this order words < mocks < Beta < product here.