Calculating the Min Entropy of a string of bits
If your probabilities are exact, then your math is right. But even if they are off by the slightest amount (e.g. $2^{-10000}$), it doesn't hold any more. For a practical system like timestamps in a computer, it's quite unlikely that your have exact values for the probabilities. If those probabilities are average measurements, then the resulting entropy is also only an average and not a lower bound (minimum).