I normally charge my Iphone during the night. Early in the day it shows the time since last full charge fine, but suddenly it starts to show nothing just a dash in the time.
These are just the normal usage and standby times.
OP, the only reason you'd have dashes is if your iPhone reset somehow or was restored. If it's only done it once, try a reboot to see if it works right. If it's doing it more than once you might want to restore it n DFU mode.