select date part (first 10 characters) of date (e.g. --ENDTC) and subtract two dates from another

[JozefAerts]

For SEND-(DART-)406, I need to take the earliest date from SJ.SJSTDTC, so I use "min_date", but then I may only use the date part (first 10 characters), and then subtract 2 such dates from each other. I can however not use the "dy" method, as that is specific for DM-RFSTDTC which excludes "0". So I need to have the "real" (not-FDA-method) difference.
I did sofar not find any operators for this.
How do I get the first 10 characters? Is there something like a "substring" operation?
Is there an operation like "datediff" that provides the "real" (not-FDA-method) difference?

Many thanks in advance,
Jozef