Persistance on datetime item

I have a DateTime item that records that last time I changed the battery in a device. I define it in a .items file here:

DateTime  LivingroomMotion_BatChg	"Last Bat Change: [%1$tm/%1$td %1$tl:%1$tM %1$tp]"		<calendar>

I persist it in a mysql db persistence here (removed all other items in this post for clarity)

// persistence strategies have a name and a definition and are referred to in the "Items" section
Strategies {
everyMinute : "0 * * * * ?"

Items {
LivingroomMotion_BatChg : strategy = everyChange, restoreOnStartup

Then I assign this a value in a rule with the following

postUpdate(LivingroomMotion_BatChg, new DateTimeType())

The rule works correctly as this item does get set to a date time when I sense a battery change and I can see that datetime on my sitemap, however the persistence is not restoring the value. For example I edited the .items file where this item is defined for an unrelated item and when this file was saved/reloaded by OH, my date/time item now shows as undefined “-” on my site map. Not sure what I am doing incorrectly here. Anyone know how to debug this?