Based on the regulations stated in the Visumhandbuch (July 2018)
(Visa Guidlines for the issuing of National Visa's)
- General conditions for the issuing of a national visa Chapter V
(1) Working visa's are (in general) to be approved by the local German Foreigners office
- where the date is then set, based on application
(2) States that the Foreigner must report to the Foreigners office before the visa expires
Based on these regulations it would seem that:
- the consulate cannot change a date approved by the Foreigners office
- any desired extension must be applied for at the Foreigners Office
Link where the Visumhandbuch (in German) can be found:
https://www.auswaertiges-amt.de/de/einreiseundaufenthalt/visa/207794
This page also states:
- A Visa, that the Foreigners Office must approve, may take up to three months
- the application should be made in a timely manor