Hi Walt,
Can you tell me more about what you are trying to do? Do you have your own weather data, or are you hoping to compare the measured production data from SAM results with a weather file downloaded from the NSRDB?
SAM reads the time stamp data from the weather file to calculate the sun angle in each time step, so if you are using your own weather data, you can have it start on any day of the year, as long as the file contains one year's worth of data.
That approach should work if you are only using the performance model. If you are using a financial model that includes time-dependent features like time-of-use electricity rates or time-of-delivery PPA price multipliers, the financial model assumes that the first time step is on Monday, January 1 for those calculations.
We do have plans to add some flexibility to SAM's time conventions -- it is a little more complicated than it might seem.
Best regards,
Paul.