Since I applied the latest release of firmware (v3.8.6) for my 2925, I notice that it doesn't seem to know of the month January. On the dashboard screen, it reports:

Current Time undefined 22 2018 14:17:40

Where as it really should be saying: Jan 22 2018 14:17:40


However if you go to the System Maintenance | Time & Date, it reports:
2018 Jan 22 Mon 14:17:40


Is this feature just in the 2925, or have they applied it to other router models too?

It's a known issue with the Vigor 2925 on the 3.8.5 & 3.8.6 firmware, it will be fixed in the next release firmware

To my knowledge it doesn't affect any other models.

This bug also affects the 2860 on 3.8.6 and the 2760 on 3.8.7.

Its a web UI issue as "sys time show" reports correctly

admin3 wrote: It's a known issue with the Vigor 2925 on the 3.8.5 & 3.8.6 firmware, it will be fixed in the next release firmware

To my knowledge it doesn't affect any other models.

Well it hasn't been fixed fir the 2925 in 3.8.7 in fact its worse, it now shows " Invalid Date Invalid" and "sys version" shows

"Router Model: Vigor2925ac Version: 3.8.7 English
Profile version: 3.0.0 Status: 1 (0xc6a0fd37)
Router IP: 192.168.0.1 Netmask: 255.255.255.0
Firmware Build Date/Time: Dec 18 2017 18:00:20
Router Name: xxxxxxxxxx
Revision: 71502 V386_2925"

Curious, your build time for for 3.8.7 is the same as for 3.8.6, but the revision number is different (as shown from my 2925 below)

wey-gwy> sys version
Router Model: Vigor2925n Version: 3.8.6 English
Profile version: 3.0.0 Status: 1 (0x482e569c)
Router IP: xxx.xxx.xxx.xxx Netmask: 255.255.255.0
Firmware Build Date/Time: Dec 18 2017 18:00:20
Router Name: xxxx
Revision: 70642 V386_2925

I'm also seeing it on 2860n, fw 3.8.6_BT
Happens whether you set NTP or Browser time