Dahua security camera surveillance DVR systems have no built-in “forgot password” or factory reset capabilities, so it is possible to lock yourself out of the DVR if you forget the password for all administrator users.
Dahua DVR systems include a secret override password that is calculated based on the system date and time. The calculation used depends on the particular DVR model. There are a number of web sites of unverifiable reputation that offer to calculate these passwords for you, for a fee. Dahua support may also provide this service for a fee. However, there are a small number of free calculators out there. Most provide only one of the possible calculations, but I was able to find one site that performs three (possibly all) calculations. For the model I was working on, the second password worked, but all other calculators I came across gave only the first option.
Enter the day and month as a 1- or 2-digit number, and the year in 4 digits, as displayed on the DVR screen. Use the generated passwords for any administrator user and once you are in the menus you can change any passwords to a new one.
The date provided must be exactly as shown on the DVR, otherwise the generated code will not work. It is also not clear if these three possibilities cover all Dahua DVRs on the market, so some models may not be accessible by this method.