When exactly are you “promoviert” in Germany?
The Promotionsordnung of my alma mater says this:
§17(1): "Als Promotionsdatum gilt der Tag der bestandenen mündlichen Prüfung."
(The date of Promotion is the day of the successful oral exam.)
§25(1): "Als vorläufigen Nachweis der Verleihung des Doktorgrades erhält der Bewerber vom Prüfungsamt der TUM eine Urkunde nach Anlage 2, sofern die erforderlichen Exemplare nach § 20 fristgerecht eingereicht worden sind."
(You receive a preliminary certificate when you have submitted the required copies of your dissertation.)
§25(2): "Vor Aushändigung der Urkunde nach Abs. 1 ist der Bewerber nicht befugt, den Doktorgrad zu führen."
(You are not allowed to call yourself a Doktor before you have received this preliminary certficate.)
§25(3): "Der Bewerber erhält ferner eine Urkunde in deutscher und englischer Sprache nach Anlage 3 a oder 3 b, die mit dem Siegel der TUM versehen ist und das Promotionsdatum gemäß § 17 Abs. 1 trägt. ..."
(The date of Promotion is written on the final certificate.)
So, you could say you are promoviert when you have passed the oral exam. But since I'm not a lawyer I'm not qualified to give legal advice. Personally, I would avoid anything that could be seen as you saying you are a doctor before you are allowed to do so. Until then I would say "I've defended my dissertation successfully and am waiting to receive the certificate." When you have your certificate, the date of Promotion is the day of the oral exam.
In practice, you get the preliminary certificate rather quickly. I believe it was something like two weeks after the exam for me, but it depends on how quickly the head of examination and the administration works. The biggest problem was getting a Führungszeugnis in time since I hadn't considered that prior to the exam.
If you want to be pedantic (and which mathematician doesn't?), the Duden (the traditional German dictionary) defines the base verb promovieren as
- a. die Doktorwürde erlangen, b. (über ein bestimmtes Thema) eine Dissertation schreiben
- jemandem die Doktorwürde verleihen
- (bildungssprachlich veraltend) fördern, unterstützen
Here, Doktorwürde is a slightly more pompous synonym of Doktorgrad (the PhD degree).
Point 1 covers the intransitive usage: a. to acquire the doctoral degree and b. to be in the process of (working towards) acquiring the doctoral degree. Point 2 covers the transitive usage: to confer the doctoral degree (on someone). (Point 3 is going out of fashion and coincides with the English cognate to promote in the sense of to support or encourage.)
Note that 1a. and 1b. are different meanings, but only one of these -- namely 1a. (or 2.) make sense as a past participle; 1a. and 2. are also the only meanings concerned with a formal status of any kind.
Hence, you are promoviert as soon as the university confers the degree, which is done (retroactively, which often leads to confusion as you point out in your question) by handing you the official diploma (or the preliminary diploma, if such a thing exists).
The faculty of natural sciences of the Leibniz-University Hannover has a slightly more elaborate page on the Promotionsverfahren on their webpages. I will be quoting from the original plus their official English translation.
Die Promotion in acht Schritten
Das Promotionsverfahren umfasst acht Schritte, die als Überblick im Folgenden skizziert werden:
[…]
Schritt 6: Mündliche Prüfung oder Disputation und Gesamtbeurteilung der Promotion
[…]
Schritt 8: Promotion
Die Promotion wird durch Aushändigung oder Zustellung der Promotionsurkunde vollzogen, nachdem Sie die Veröffentlichung der Dissertation nachgewiesen haben und nachdem Sie nachgewiesenermaßen alle Ressourcen zurückgegeben haben, die Ihnen für Ihr Promotionsprojekt vorübergehend zur Verfügung gestellt worden sind.
Eight steps to obtaining a doctorate
The procedure for obtaining a doctorate has eight steps, which are outlined below.
[…]
Step 6: Oral examination or thesis defence and overall assessment of the doctoral research
[…]
Step 8: Conferral of doctorate
The doctorate shall be conferred by handing over or delivering the doctoral diploma after you have proved that the doctoral thesis has been published and provided proof that you have returned all the resources which were temporarily made available to you for your doctoral project.
This webpage makes it pretty clear that the faculty of natural sciences of the Leibniz-University Hannover considers Promotion (and thus, although it is not explicitly mentioned, likely also promovieren) to cover the entirety of your steps 1 to 5.
Furthermore, the requirements for the reverse of the title page additionally state that the Tag der Promotion is considered to be the day of step 6 according to the website or step 3 in your overview.
I assume this is because the Promotion is considered to be the entirety of the process, while anything that happens after the defence is considered automatically happening.