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You should take advantage of having one clean timeline for all your books and use the date system. The date system should indicate starting dates.
yyyy.mm.dd - yyyy.mm.dd" style date ranges destroy the advantages of yyyy.mm.dd notation. Use yyyy for a product that spans more that a years time and yyyy.mm for a product that spans more that a months time but less than a year. Obviously this is less accurate than a range, but it's better overall.
Book and Series labels are completely unnecessary in a date system that catalogues products in a single universe and timeline. Book and Series labels should only be used as a marketing tool to contextualise a product as it is being released. Fans will adopt these labels on wikis etc.
You do not need to preface the date with "Siglerverse" unless you intend to use the same dating system on future products that are set outside the Siglerverse.