You can't use the DAYSEQ() function in a DIGEST-H or INDEX-H file.
Please note that there is an ERROR in the Site Manager's Manual in
chapter 10.7; where it reads

"You use the same formatting commands and replaceable parameters in
the DIGEST-H and INDEX-H files as you do in the MAILTPL file, and the
instructions for storing them on the server are identical."

it SHOULD read:

"The DIGEST-H and INDEX-H files are plain text files, like the WELCOME
and FAREWELL files, and the instructions for storing them on the server
are identical.  Note that, as with the WELCOME and FAREWELL files, you
cannot use the template formatting commands and replaceable parameters
discussed above."

I will make this change in the Site Manager's Manual as soon as I can.
This error exists ONLY in the Site Manager's Manual; the List Owner's
Manual has the correct wording in its equivalent chapter (9.6).

Apologies for the confusion.

Nathan