Costing variant stored with start of period

What causes this issue?

In Customizing for Product Costing, costing type &V2& is
assigned to costing variant &V1&. There, the costing type was set to be
saved with the start of the period for manual costing. It is,
therefore, not necessary to make an entry in field "Costing date".

System Response

Any entries in this field will be deleted by the system after you press
ENTER.

Error message extract from SAP system. Copyright SAP SE.