Mandays are pointless. Mandays are bad. Mandays don't exist.

Here is the dictionary definition of a manday:

An industrial unit of production equal to the work one person can produce in a day.

It's a poor way to quantify a programmer's output because it's a variable with a huge range. Time-based performance measures in software development are flawed and kind of old school. Same with Lines of Code (LOC). As Negroponte recently pointed out in his AIGA conference speech,

If programmers were paid by the amount of lines of code they removed as opposed to added to a piece of software, the world would be a better place.