Master Calendar is basically a Date Dimension, in which you create all the required fields/flags you may need on the front end calculations. e.g. YTD (Year to Date), MTD (Month To Date), Week Number etc.. Once you have these flags all ready in script itself, its very easy to answer many questions easily on the front with simple set analysis or formulae.
Now, which date you will be using to create your master calendar? -- this totally depends on what business process is it you want to analyse in front end across date/time dimension.
But there is no limit of creating Role Playing dimensions in Qlikview. Lets say, you created a master calendar based on the min and max date taken from Joining Dates. you can re-use the same logic (after renaming stuff), for Date of Resignation etc.
If you need the actual script, you may want to take a look at the Josh Goods article:
