Parâmetros Bash não são expressões regulares, são globs. Use:
$ zip /var/www/html/db-$(date +\%F-\%T).zip /var/www/html/db-*.sql
Do manual de bash , em "Correspondência de padrões":
*Matches any string, including the null string. When theglobstarshell option is enabled, and*is used in a pathname expansion context, two adjacent*s used as a single pattern will match all files and zero or more directories and subdirectories. If followed by a/, two adjacent*s will match only directories and subdirectories.