Fix replace(tzinfo) bug

[Ariana-B]

Have timeline dataset counter convert day to datetime instead of date to make replace(tzinfo=None) valid.

Testing locally, localhost:8080/api/dataset-timeline/ga_ls5t_ard_3/1991/11/21 now downloads a json instead of throwing a server error.

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:books: Documentation preview :books:: https://datacube-explorer--489.org.readthedocs.build/en/489/

[pindge]

can you add a ticket to have a test case for this setup? either unit or integration is fine,

[Ariana-B]

@pindge there's already a test for the timelines; I've added in a test case for year/month/day

[jeremyh]

Thanks for looking at this --- I tried cherry-picing this change to the current stable branch, but it looks like it breaks the product page (and a bunch more tests). See #492