The white tip on the rear fender pre-dates WWII. The popular conception is that the white tip originated in WWII, but it is actually an older requirement. On 18th October 1934 under the "Road Traffic Acts 1930 to 1934, the Pedal Cycles (White Surface) Provisional Regulations". This specified a white patch of 12 square inches, centered 13 inches above the ground. An earlier requirement of a rear reflector dated to 1927. However, some bicycles had made use of the white tip paint prior to the 1934 regulation, dating back to WWI. It was apparent even before 1934 that a reflector and white paint was somewhat helpful in dim conditions.
However, this requirement was a UK requirement, and some of the bikes built particularly for export did not have the white tip.
In some cases, the fender was painted all-black and then an add-on plate painted white was added on top of the all-black fender. This practice lasted well into the 1940s as it allowed the manufacturer to select when they wanted to add the white, and when not by simply bolting or not bolting on the add-on plate. But most white tips were painted directly on the fender. I've seen videos of a factory in India that paints the white tip by dumping paint from a can.
It's hard to date a bicycle solely on whether it has a white tip or not. You'd have to use hub date, frame features, graphics on the frame, equipment, plating type, etc. to date overall. We sometimes see people claiming a "pre-1934" or a pre-WWII bicycle based solely on lack of a white tip on the rear fender, when the bicycle in fact turns out to be newer but simply built for a market where the white tip was not required.
And then sometimes it's simply a case of the white rubbing off, or over-paint by the end user. I have a very old (well by US standards) Birmingham Phillips roadster that had a white tip where the paint was degraded and wiping off. I re-painted the white portion with an "aged white" color. I suspect the bike was from right before WWII.