Use assertions to ensure that dates make sense and change the implementation of operators <= and >=, increasing the readability

Clang-tidy complaints, so the if-else structure of the implementation of the operators changed. The code is a little bit ugly so it should be evaluated

Fixed a bug: In utcTime function, when the year was leap year, the extra day was substracted two times instead of one.

Added a function that converts elapsed seconds since Unix epoch to UTC date and the mission-specific function has been implemented