input%time_index: return 0 for start-time of the file as special case

For chunked reading, we usually determine the time-offset by calculating the time index of the start time. If the start-time is the same as the start-time of the input file, the index should be 0.

It was 1 before, but that leads to a read out of bounds in the chunk.

With this MR we treat the start-time as a special case for the time index and return 0 in that case.