If you update to Joomla 3.8.10, please make sure that you either do a full GitHub update of Fabrik, or disable caching for your lists (in the Advanced settings) or turn off Joomla's System Cache setting. A change in Joomla's cache code in 3.8.10 required a fix in Fabrik, which will be rolled in to the next release, but is currently only available in GitHub.
Days between 2 dates in PHP
This gives the difference in days between a date element and today, Alternatively you could swap 'today' with another date or field
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