PRS-T1 Loose time under sleep mode

For almost a year now I own a PRS-T1 (not rooted) but recently I have noted that the reader loose time when I put it in sleep mode (standby mode); it occured evertime and never recovered right time when I resumed the reader.

I have also installed the latest firware upgrade.

So, is there some (undocumented) feature to keep time under sleep mode?

:thanks: