Multiple references to the same author but without a date In a thesis using the “(Author, Year)” citation style a problem arises when the date is missing. Then Citavi for Word just writes “(Author)”. Now in case that there are several undated sources from the same company, all of them become “(Company)”, which is ambigious. If there was a year one could simply do “(Company, 2019a)” and “(Company 2019b)” to resolve this.

What shall one do when there is a missing date and there are multiple sources? Shall one just use “0000a” and “0000b” for the date?
You can use the same approach as for multiple sources from a single known year, adding an additional letter. For example one way of writing it would be "(Company, n.d.-a)" and "Company, n.d.-b)", where 'n.d.' is short for "no date". Adapt as needed to meet the requirements of your citation style. The hyphen is required for the APA style, for example, but might not be universal:


  Also remember that if you have two or more “in press” or “no date” references with the same authors in the same order, you should use lowercase letters—a, b, c, and so forth—after the publication date and alphabetize the references by their titles (excluding A, An, and The; see p. 182 in the Publication Manual). The only difference between these types of references and references with publication years is that “in press” and “no date” references contain a hyphen before the a, b, and so forth:
  
  American Speech-Language-Hearing Association. (n.d.-a). The knowledge . . .
  
  American Speech-Language-Hearing Association. (n.d.-b). A strategy to . . .
  
  Schafer, G., &amp; Plunkett, K. (2011a). The power . . .
  
  Schafer, G., &amp; Plunkett, K. (2011b). Task complexity . . .
  
  Schafer, G., &amp; Plunkett, K. (in press-a). The rapid learning . . .
  
  Schafer, G., &amp; Plunkett, K. (in press-b). Sometimes a child . . .