Worker surprised by manager's reaction when they give their 2 weeks' notice, manager demands that they stay longer and give 1 month total: '[My] manager is upset that I didn't give "advanced notice"'

When it comes to hard decisions being made by the organization, like layoffs or budget cuts, the reason given is always that "It's just business," but when that shoe is on the other foot, suddenly that same cold, hard fact vanishes, and employees are suddenly expected to "Do the right thing." 

As it is, a two-week notice is a sign of mutual trust and respect. It's a professional courtesy, and seldom actually a contractual obligation. And it's risky too, as in a lot of employment jurisdictions, there is nothing to stop your employer from firing you the moment they know you have an intention to leave.

People can be unpredictable, too. A long and healthy business relationship between an employee and manager/employer can be shattered in moments when the employee expresses their desire to leave. A feeling of rejection and betrayal, and sometimes the shattered illusion of control over the employee, can drive the manager or employer to seek to end things on "their own terms," even if it is cutting off their own nose to spite their face. That sense of control being restored, their ego protected, being worth it to them, the cost of business.

And these types of reactions really don't make sense either. Hiring processes take time and have a high cost in the labor needed to successfully find and vet the right candidate, and they have an even higher cost in the consequential loss of reduced productivity while the role remains unfilled. So, rightfully so that a manager would want to have as much time as possible in order to make it work. 

However, just as employees need references, employers should also build themselves a reputation of having done the right thing for previous departing employees, thus letting future staff know that they are safe to give long notice periods. Thus, in turn, by showing that they can react professionally to employee resignations, they reduce the cost of hiring.

For this reason, giving any notice period is a risky move, and although they would like to say something sooner, they often wait until the last possible moment, following what they feel is the bare minimum necessary to still be able to get that reference (if they even want it from this employer at all.)