I would like to know how the calculation is happening for DatedConversionRate's nextstartdate, is it depends on some configuration setting?
This value is simply whatever the next DatedExchangeRate's StartDate is. For example, if you insert a record with a StartDate for 2020-01-01, then you insert another record for the StartDate 2021-01-01, the NextStartDate for 2020-01-01 will be set to 2021-01-01. If you later add a record with a StartDate of 2020-06-01, then the NextStartDate for 2020-01-01 will change to 2020-06-01, and the NextStartDate on 2020-06-01 will be 2021-01-01. This is all done by the server, so you don't need to worry about managing this.
This is a feature of Advanced Currency Management, where you can set exchange rates for a particular time period.
It includes a link to more detailed information on dated exchange rates.