Hello again @Afromum,
Thank you for waiting, I wasn't able to **** at the platform during the Holidays.
Hopefully you had a nice time as well!
As for your question: the BAMF website offers a lot of information regarding parental leave, parental allowance and parental allowance plus (Elterngeld/ Elterngeld PLUS). Follow this link here.
A German source I found states that
"Elterngeld kann innerhalb der ersten vierzehn Lebensmonate eines Kindes beantragt werden. (...) Rückwirkend wird Elterngeld nur für die letzten drei Lebensmonate vor Beginn des Monats gewährt, in dem der Antrag bei der Elterngeldstelle eingegangen ist. Da man sich beim Elterngeld an den jeweiligen Lebensmonaten des Kindes (nicht an Kalendermonaten) orientiert, muss ein Antrag also spätestens am letzten Tag des 4. Lebensmonats des Kindes einen Eingangsstempel von der zuständigen Behörde erhalten, damit der Anspruch auf Elterngeld rückwirkend ab Geburt des Kindes gilt."
Translated to English: You need to apply for parental allowance within the first 14 months since the child's birth. Retrospective payments are possible for the three months prior to the application. That means - if you want to be able to receive parental allowance since the day your child was born (retrospectively), the application would need a post stamp ("Eingangsstempel" = arriving date), given by the responsible authority, latest on the last day of the fourth birth month of the child.
The websites I've been looking at advise you to speak to a local "Elterngeld" center. On this website called Elterngeld.net, you can choose the Federal State and find the closest one (it's in German). The website itself offers counselling and support with filling out all documents, but they charge money for it.
I hope this answers your question!
Let us know if you need further assistance with anything.
All the best,
Isa