Implement middle date between two dates

[pounard]

Following #8.

See:
SQLite: https://stackoverflow.com/questions/58721481/how-find-the-middle-point-between-2-dates-in-sqlite
SQLServer: https://stackoverflow.com/questions/12745414/sql-how-to-retrieve-the-middle-point-between-two-given-dates
PostgreSQL, MariaDB and MySQL will be relatively easy.