Great question — answering as the UphillTech team:
If anyone else encounters this, check your source code and try and find the date in question to see where it's being inputted, Googlebot normally finds these dates there. It turned out the Googlebot was picking up a date in the footer of my site, via the WordPress Latest Posts Widget, where I had selected the option to feature the Post date. Resulting in this date showing in the source code of every page throughout the site.
Full answer by Tom25: https://wordpress.stackexchange.com/a/218123 (CC BY-SA 3.0).
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