If the proportion in the first interval is less than .50, then I think it would be best just to say that the median duration is <1 month. If the proportion in the final interval is more than .50, then it would be best to say that the median duration is greater than the final month in your data.

The DHS calculation is not simple. It may change in the future to use day of birth, not just the month and year of birth (and the surveys always give the day, month, and year of interview). If you are having difficulty matching the DHS calculation, please tell us what survey you are using.