Has the battery percentage indicator been acting up lately on your iPhone 6s or iPhone 6s Plus? If so, you're not alone: this is a known issue, according to Apple's newly published support document.
“If you change the time on your iPhone 6s or iPhone 6s Plus manually or change time zones when traveling, you might notice that the battery percentage doesn’t update,” reads the document.
Apple is aware of the problem and working hard to deliver a permanent fix.