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On it's own, Delivery Confirmation is not dated. You can add postage and mail whenever (within a couple weeks, or the system may not retain the tracking number). So yes, you can buy it from PBS today and mail it tomorrow, or the next day, etc.
Printable postage (such as PBS postage) is dated and must be used on or before the date or you run the risk of having it returned for 'stale' postage. The PBS postage printing system allows members to change the date, such as two days from now, to allow time for mailing if you need it. When in doubt, choose a date in the future b/c we hear USPS will accept pre-dated postage but not late dated postage.
Postage is dated, Delivery Confirmation is not.