Greetings all, some Excel issues that my noggin cant handle...

I have a date (totally random 12.12.12 for example), and I have a value "k" (any number, like 7 for example).
Ive been tasked with some things that I cant quite figure out what to do:
1) First, I need to figure out the original dates next months day which is "k". I figure something along the line of adding a month and the value of k to the original date. But how do I add months specifically ? If I even grasped the idea correctly.
2) I now need to give the newly made date in point one the day name. the function: =IF(WEEKDAY(C11)=1;"Monday";IF(WEEKDAY(C11)=2;"Tuesday";IF(WEEKDAY(C11)=3;"Wednesday";IF(WEEKDAY(C11)=4;"Thursday";IF(WEEKDAY(C11)=5;"Friday";IF(WEEKDAY(C11)=6;"Saturday";IF(WEEKDAY(C11)=7;"Sunday"))))))) Wont do, I need to take the dates from a list, with 1 being Monday, 2 being Tuesday and so forth. The first function doesnt work because it appears to take the day name from a calendar from somewhere. As in lets say day 2 on list is Tuesday, but with that function above it could be any date from the calendar. Also, how can I make sure that in the list day 8 is Monday, day 22 is Tuesday...
3) I am also required to substract days from one date from another. Like 21.05.2080 minus 20.02.2014 equals ? in days... and just out of my own curiosity, how could I extract months... This shouldnt be too hard, but I still managed to screw up It gave me the value in some random date 19.05.1900...

Thank you for taking your time to read this and perhaps even possibly replying with a few helpful tips.