dotnet.System.Convert should work if you add 'System to het CRR

CRR can be reached hitting SHITF+F7
change 'Type' to '.net'
then hit 'F5' on the column 'Name', and pick 'System' (versie 4.0.0.0) from the list.

now you should be able to update a Date field with this expression:
DVAL(dotnet.Convert.ToDateTime("5-Jul-18").ToString(),'yyyy-MM-dd')