Java generate and store half hourly time steps from start to end date

Dear all,

I'm trying to convert an excel model into java.  I need to generate half hourly time steps based on a start and end date provided by the user and store into a data structure.  I've converted the date into simple date format dd-MM-yyyy.  What is the most efficient way to generate this data and store in the data structure below?

I've decided to store the data in nested Maps, i.e. (if there is a better way let me know...).  Weather is an example of one set of data, there are 5 others that will need to be accessed in the same way.

Map<Date, Map<String, double>> weatherMap = new HashMap<Date, HashMap<String, double>>();

The inner map contains the timestep as key, and weather data as value.  The outer map will contain the date as key and the inner map as value.

I've defined the timestep value as type string, not sure if it should be double?

Thanks