Via an ingest pipeline using a processor such as a script (similar to this) or another processor is it simple to add a field with the date x days (a field on the ingested doc) after its ingested.
Such that a doc
field: somedata SLA: 14
has a field 'overdue' that is the date x days (in this example 14) from when its ingested
field: somedata SLA: 14 overdue: x/x/21