There's a saying, "You can't fit ten pounds of (crap) in a five-pound bag." Your job is to show that the task is ten pounds and they're asking to have it in a five pound timeframe.
The only thing you're missing is the time estimates. Put a time estimate on each task, and show how all these things add up together to the estimate you provide. Don't allow any estimate to be larger than 4 hours. If you have any task where you say "a day" or "10 hours", then break it down into smaller subtasks.
2h make some changes to Database
2h add front end HTML
write server side code
4h input validation
4h database inserts
2h add validation
use unit tests
2h client-side tests
3h server-side tests
2h make sure SEO is setup is working
2h implement email confirmation
2h optimize DB changes for speed
2h refactor and optimize the code for speed
Now you've got an itemized bill of the costs. All told, that totals up to 27 hours of work.
You can now show this to your customer and say "These are the things that must be done, with the cost of each." Use the word "cost", because time IS a cost, and management understands costs. Explain that you could possibly drop the two optimization tasks on the end, but they'll have a negative effect down the road, and they're only 15% of the total estimate.
Also make sure that you explain what your hours/day is, realistically. For example, if you're called on to do tech support, or maintain databases, or whatever, figure that into your estimate. Don't say "Well, I can do 7.5 hours a day of good coding" because you probably can't. It's probably more like 5 or 6.
Then, most importantly, track your progress. Say that you can do 5 hours per day of coding. Then you should be able to knock off the first two tasks (in my example) on Monday, finish the third and start the fourth on Tuesday, and so on. Make a checklist that shows this, so that you can show them on Wednesday when they come by and say "How's it going are you still going to be done by the end of Friday?"
See my slides for my talk Preventing Crisis: Project Estimation and Tracking That Works that I gave at OSCON a few years ago. Look at slide 21, "Planning the week". There's also a sample velocity chart.