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Have you checked the nature of your custom field? Are you sure is it a date?. It could be a date in string format, but not a data data type. Try to invoke a date function in your custom field to see if it really a date. For example:
{{[date].plus[Unit]([number])}}
For example:
{{customfield_10008.plusDays(1)}}
and to log it and see if the result is the expected.
