When a smoker sets a date to quit, it can sometimes be effective. But if you try to set that date for them, it can backfire. Demanding someone quit all at once, cold turkey, is one thing, but pushing a smoker to agree to an imposed date likely won't work. When the all-or-nothing date is set by the person who smokes, it's much more likely to succeed and your help in the process will be more welcome because you'll be supporting, rather than pushing, the timetable. Many smokers do go smoke-free for their loved ones, but even then the motivation comes from the individual and not from outside.
And if their goal date comes and goes and they are still smoking or relapse, acknowledge their effort anyway. Although it may seem slow-going and an unachieved objective, calling them a failure takes away from the progress they've already made, which could cause them to ramp back up to smoking just as much as before. Showing a great deal of disappointment when smoking continues, especially if they are smoking less, only reinforces any thoughts they may have that they can't succeed at quitting.