Overtime can be calculated on either the basic salary plus DA (dearness allowance) or the gross salary, depending on the company's policies and the applicable laws and regulations.
In some countries or industries, there may be specific laws or collective bargaining agreements that govern how overtime is calculated. In other cases, the company may have its own policies on how to calculate overtime, which may vary from one company to another.
Generally speaking, overtime pay is usually calculated as a multiple of the regular hourly rate, with the specific multiple depending on the applicable laws or company policies. For example, in some countries, overtime may be paid at 1.5 times the regular rate, while in others it may be paid at double the regular rate.
So, whether overtime is calculated on basic salary plus DA or gross salary, the method used will ultimately depend on the specific laws, regulations, and policies that apply in the relevant jurisdiction or company.