issue #8 - date add/sub and date interval support #69

Job	Run time
PHPUnit with PHP version matrix (8.1, 16, pdo_pgsql, lowest)	2m 13s
PHPUnit with PHP version matrix (8.1, 16, pdo_pgsql, highest)	2m 16s
PHPUnit with PHP version matrix (8.2, 16, pdo_pgsql, lowest)	2m 17s
PHPUnit with PHP version matrix (8.2, 16, pdo_pgsql, highest)	2m 17s
PHPUnit with PHP version matrix (8.3, 16, pdo_pgsql, lowest)	1m 54s
PHPUnit with PHP version matrix (8.3, 16, pdo_pgsql, highest)	2m 38s
PHPUnit with MariaDB (8.1, 10.11, mysqli)	1m 41s
PHPUnit with MariaDB (8.1, 10.11, pdo_mysql)	2m 13s
PHPUnit with SQL Server (8.1, pdo_sqlsrv, mssql)	2m 14s
PHPUnit with PostgreSQL (8.1, 10, pgsql)	2m 12s
PHPUnit with MySQL (8.1, 8.0, mysqli)	2m 5s
PHPUnit with MariaDB (8.1, 11.0, mysqli)	1m 59s
PHPUnit with PostgreSQL (8.1, 10, pdo_pgsql)	2m 15s
PHPUnit with MySQL (8.1, 8.0, pdo_mysql)	1m 53s
PHPUnit with MariaDB (8.1, 11.0, pdo_mysql)	2m 7s
PHPUnit with PostgreSQL (8.1, 15, pgsql)	2m 23s
PHPUnit with MySQL (8.1, 8.2, mysqli)	2m 18s
PHPUnit with SQLite (ubuntu-22.04, 8.1, pdo_sqlite)	1m 11s
PHPUnit with PostgreSQL (8.1, 15, pdo_pgsql)	2m 14s
PHPUnit with MariaDB (8.1, 11.1, mysqli)	2m 19s
PHPUnit with MySQL (8.1, 8.2, pdo_mysql)	2m 35s
PHPUnit with SQLite (ubuntu-22.04, 8.1, sqlite3)	1m 5s
PHPUnit with PostgreSQL (8.1, 16, pgsql)	2m 14s
PHPUnit with MariaDB (8.1, 11.1, pdo_mysql)	1m 44s
PHPUnit with PostgreSQL (8.1, 16, pdo_pgsql)	2m 7s
	52m 24s

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