In a prior post here, part of my statement included:
" To calculate would be overly complicating it, ..."
That was a poor choice of wording. Should be: use the method that provides results most closely matching the actual option pricing. If using available interest rate time increments is best fit, use that (simpler) if using interpolated time values for interest rate is better fit, then by all means use that. It is trivial to do, but should insure, if possible, it will more correctly reflect actual option pricing. Since I do not have the answer, I chose the simple method until I discover otherwise. I did not want to increase my error by assuming interpolated interest rates were more accurate for option pricing with BSM.