You are seeing this because the
wtmp file is rotated at the 1st of each month. (e.g. wtmp is moved to wtmp.1 and a new wtmp is created empty).
In the Raspberry PI, as you do not have a Real Time Clock to keep the time, each time you (re)boot it you are back to Epoch 0 which is Jan 1, 1970.
The good news is that you can buy an RTC (DS3231) for the Raspberry in AliExpress for less than 2 Euros, or in Europe for around 5-10 Euros The Pi Hut. I myself bought one for my Lamobo R1 and another one for my rpi 3.
see Adding a Real Time Clock to your Raspberry Pi
As a side note, if having an Internet connection, the rpi is supposed to get back in time sometime after booting via the NTP protocol; however having an RTC means it will get back in time earlier in the bootup sequence; also the RTC is particularly handy for Raspberries/Arduinos that are not connected to the Internet.