1Hz = One Cycle and One Cycle = 360degrees
When you want to find the difference caused by a time separation of two waves, you can find how much of a wave cycle will pass during the time delay.
At 0.25Hz, it takes 4 seconds to make 1 Cycle. With some algebra you find that 1 cycle every 4 seconds = a quarter cycle every 1 second.
So now that you know a one second delay causes a quarter of a cycle to pass, you can convert that into degrees by multiplying 0.25 and 360. 0.25x360=90
Lo and behold, a time difference of 1 second, creates a phase seperation of 90 degrees.