How to determine date of last Android system update?

Hi,
Is there a way to programmatically obtain the date when the last Android system update was installed on the device? I know the build time can be obtained either from reading Build.TIME or ro.build.date, but this isn't when the update was actually installed. I also tried looking in my Nexus 5 for system file timestamps, but they all seem to be from 2009 or 1970.

Thanks.