The README was erronously showing all timestamps as being of String type but they are defined as DateTime type in the things definition file.
If you change your timestamp items types to DateTime and use the following formatting:
DateTime SmartLockLastUpdated "SmartLock Last Updated [%1$tY-%1$tm-%1$td %1$tR]"
you will hopefully get consistent formatting.
This page is a good source for different DateTime formatting options.