Don't travel back to Jan 1. That forces you to keep time traveling forward even if your rehab is done. Start off from the current date and travel forward one day at a time. Each day, plant all the flowers Nook is selling along the edges of your pathways, walk around watering flowers, and talk to all your neighbors (because if one of your neighbors moves out, it makes a big dirt patch where their house once was). Once you achieve the level of grass restoration you want, TT back to the current date. Here's why you do it in this order: because basically all the damaging effects of time traveling occur when jumping forward, so you want to make the forward jumps as small as possible and take time to water everything and reduce the damage caused. When you're done, TTing backwards to the current date essentially has no negative side effects.