Comment by BrandoElFollito

The problem when building it yourself is that this is usually done by "generic" developers who discover edge cases (often together with the requester) that threaten the whole model.

A company doing payroll for us ("us" being a multinational company) asked for a "typical payroll" to start with. Fortunately we had experienced people on the pay side who discarded the company because of this question (for one they should know, and for two they should know that "typical" will cover maybe 60% of the cases -- I thought that this was an exaggeration until I discovered the reality of calculating pay in France)

A good company specializing in "pay" or "vacation" (which are very regulated over here) will know the "typical" case and the edge cases.