return None for invalid date
return None for invalid date

spatialdev/onadata,jomolinare/kobocat,makinacorpus/formhub,SEL-Columbia/formhub,GeoODK/onadata,GeoODK/formhub,qlands/onadata,GeoODK/formhub,qlands/onadata,GeoODK/formhub,SEL-Columbia/formhub,mainakibui/kobocat,makinacorpus/formhub,eHealthAfrica/onadata,hnjamba/onaclone,jomolinare/kobocat,ultimateprogramer/formhub,ultimateprogramer/formhub,kobotoolbox/kobocat,eHealthAfrica/formhub,awemulya/fieldsight-kobocat,piqoni/onadata,GeoODK/onadata,mainakibui/kobocat,smn/onadata,piqoni/onadata,sounay/flaminggo-test,wesley1001/formhub,jomolinare/kobocat,hnjamba/onaclone,hnjamba/onaclone,hnjamba/onaclone,spatialdev/onadata,ehealthafrica-ci/onadata,ehealthafrica-ci/onadata,kobotoolbox/kobocat,kobotoolbox/kobocat,SEL-Columbia/formhub,ehealthafrica-ci/formhub,ultimateprogramer/formhub,ehealthafrica-ci/formhub,wesley1001/formhub,sounay/flaminggo-test,spatialdev/onadata,SEL-Columbia/formhub,awemulya/fieldsight-kobocat,awemulya/fieldsight-kobocat,eHealthAfrica/onadata,ehealthafrica-ci/onadata,GeoODK/onadata,sounay/flaminggo-test,smn/onadata,eHealthAfrica/formhub,jomolinare/kobocat,kobotoolbox/kobocat,makinacorpus/formhub,wesley1001/formhub,piqoni/onadata,qlands/onadata,ehealthafrica-ci/formhub,GeoODK/onadata,ehealthafrica-ci/formhub,smn/onadata,wesley1001/formhub,makinacorpus/formhub,sounay/flaminggo-test,ultimateprogramer/formhub,eHealthAfrica/formhub,ehealthafrica-ci/onadata,eHealthAfrica/onadata,mainakibui/kobocat,awemulya/fieldsight-kobocat,spatialdev/onadata,smn/onadata,mainakibui/kobocat,qlands/onadata,piqoni/onadata,eHealthAfrica/formhub