Device: OnePlus One, non-rooted, stock Cyanogen OS
I installed an OTA update today (27 August 2016).
Now, the Android security patch level shows a date from the future (1 September 2016).
Why is it such a date?
GiantTree's explanation sounds plausible but there is a bug report raised on first of this month for a patch released last month with similar concern : Issue 218947: Security Patch Level shows the future date?
There has been no revert on this from Google, so the cause is uncertain. OP may star the bug report to be updated by Google