Within the quotation, go to the More Information > tab; "Validity Date", and you can manually enter a date, so that it will be shown in the document when viewing it in the document data section.
.If you want that by default the document shows a validity date when you view it you have to indicate it in the document template. To do this go to Settings > My Company > My Templates and within the desired template, in the Document Data tab, you must indicate the date in the "Validity Date" field, for example: Valid until: DD/MM/YYYYYY.
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