RememberDatePickerState crashes with kotlinx-datetime:0.7.1 because Compose Multiplatform Material3 still depends on an older unshaded version of kotlinx-datetime, so mixing in zero.7.X reasons runtime conflicts on Desktop/iOS. The safe workaround these days is to stay with kotlinx-datetime:0.6.X until Compose updates its dependency. You can still convert selectedDateMillis to a readable string manually the usage of platform-specific formatters (count on/real with SimpleDateFormat on Android and NSDateFormatter on iOS).