Turns out there
is a way to check this, although it requires you to know how to use inspect elements (not too complicated, but intimidating at a glance):
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Most desktop browsers have a shortcut to enter developer mode/inspect element mode (for Windows + Chrome, this is pressing F12). Then you just inspect the timestamp in your inventory (or someone else's, if the collectible is visible there) and the actual data should list the time (both in normal datetime format, and in the Unix Timestamp format).
For chrome, the button to inspect element looks like this:
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