mathing functions, datetime correction sind 0000-00-00 ist not valid vor php DateTime, take something like 1970-01-01 instead
test-data command to create empty drivingPools for 1 month:  php app/console project:testdata-drivingpools
test-data command to create empty drivingPools for 1 month:  php app/console project:testdata-drivingpools
check if workingMonth already exists