Time is a tool we use to measure motion, it is a dimension not in the same sense as length, width, or height, but rather it is used to show movement within the three named dimensions. Einstein used two light clocks, photons traveling between mirrors, in his ideal thought experiment. One clock stayed on earth with the first twin, the other clock left earth at the speed of light with the second twin. The earth bound twin sees his brother’s clock as slowing down, not because of a change in time rather a change in the timing between the two clocks. The earth bound twin sees the photons from his brothers clock first when the wave hits him after being emitted and second when the wave reaches him after the photons change directions at the top of the clock. The earth bound twin’s clock shows more time has elapsed between these sightings as his brother speeds away. Einstein called this difference between the clocks time dilation and proposed that only the second was affected. Using basic math to keep a ratio constant whenever you change the divisor; you must also change the dividend by the same amount. i.e. the expression 1/1, if you dilate the bottom number to a two it would be 1/2 which changes the overall amount of the ratio so to keep the ratio equivalent you must dilate the top number by the same amount which in our example would give you 2/2. To make Einstein's theory consistent with the math I learned as a boy if you dilate the second (in the ratio meters per second) you must also dilate the unit used to measure the distance traveled (meter) so that each clock would appear to each twin as still working correctly relative to themselves. By increasing the length of our unit of measurement we will be keeping the speed of light constant at 300,000k per second for each of the twins. By dilating the second only you are just showing the difference between the twins clocks but you are not showing why each twin still sees themselves as being unchanged. Time doesn’t decrease or increase with speed motion does, because clocks are how we measure motion then is stands to reason that it is our clocks and how they measure motion that increases or decreases with velocity.