HackerRank — ProjectEuler+ contest solutions

My HackerRank profile can be found here: HackerRank/Dalimil

ProjectEuler is a website dedicated to a series of computational problems intended to be solved with computer programs. It includes over 500 problems (July 2015).

HackerRank is a company that focuses on competitive programming challenges. Here is the ProjectEuler+ contest.

People from HackerRank have modified the original ProjectEuler challenges and turned them into harder versions by increasing input size, adding hidden test cases, and setting time limit for your programs. They have redesigned the first 100 challenges so far (July 2015).

A good example might be Problem 19: Counting Sundays

Original ProjectEuler problem statement:

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Modified HackerRank's ProjectEuler+ problem statement:

How many Sundays fell on the first of the month between two dates? You get the two dates (they are in format D/M/Y) as your program's input. The year can be any number between 1900 and 10^16 = 10000000000000000.

DO NOT copy and paste these solutions to solve the ProjectEuler challenges, as you will learn nothing by doing that. Although, I encourage you to look at my code if you get stuck and need some hints, or simply want to compare your solution with someone else.