thats cause the date is part of the URL. so changing the date changed the URL, and so thereby violated nr 2 of the Cool URL rules:

URL-Rule 1: unique (1 URL == 1 resource, 1 resource == 1 URL)

URL-Rule 2: permanent (they do not change, no dependencies to anything)

URL-Rule 3: manageable (equals measurable, 1 logic per site section, no complicated exceptions, no exceptions)

URL-Rule 4: easily scalable logic

URL-Rule 5: short

URL-Rule 6: with a variation (partial) of the targeted phrase

URL-Rule 1 is more important than 1 to 6 combined, URL-Rule 2 is more important than 2 to 6 combines, … URL-Rule 5 and 6 are a trade-of. 6 is the least important.