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Problem in Zenphoto version 1.6, both fresh install and upgraded versions, with different themes (can't say if it was introduced before)
In Archive photos are grouped under wrong months - offset by 1.
For example, 2018 starts with December, which displays photos taken in January 2018 (based on metadata), then it's January and shown are photos from February 2018 and so on.
News though are not affected - right months are listed for times posted
Known issues with dates formatting cause by PHP core changes:
Please try 1.6.1a (on a non production site) where this is hopefully fixed now.
Tried 1.6.1a and it worked great, dates are correct now.