data structure to find values in an interval

Question

I have a multithreaded simulation code doing some calculation in discrete time steps. So for each time stamp I have a set of variables which I want to store and access later on.

Now my question is:

What is a good/the best data structure given following conditions?:

• has to be thread-safe (so I guess a ordered dictionary might be best here). I have one thread writing data, other threads only read it.
• I want to find data later given a time interval which is not necessarily a multiple of the time-step size.
  E.g.: I simulate values from t=0 to t=10 in steps of 1. If I get a request for all data in the range of t=5.6 to t=8.1, I want to get the simulated values such that the requested times are within the returned time range. In this case all data from t=5 to t=9.
• the time-step size can vary from run to run. It is constant within a run, so the created data set has always a consistent time-step size. But I might want to restart simulation with a better time resolution.
• the amount of time stamps which are calculated might be rather large (up to a million may be)

From searching through the net I get the impression some tree-like structure implemented as a dictionary might be a good idea, but I would also need some kind of iterator/index to go through the data, since I want to fetch always data from time intervals. I got no real idea how something like that could look like ...

There are posts for finding a key in a dictionary close to a given value. But these always include some look up of all the keys in the dictionary, which might not be so cool for a million of keys (that is how I feel at least).