With a lot of work I managed to do:

{ SET Delay -1 } { SET m { =MOD({ DATE \@ MM }+Delay+11;12)+1 } }{ SET y { =INT({ DATE \@ yyyy }+(Delay+{ DATE \@ M }-1)/12) } }{ SET MyDate "1/{ m }/{ y }" }{ MyDate \@ "d/M/yyyy" }

Thanks all!

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Backup your original file before doing any modification.