The phrase "cooler times" refers to a period in the past when things were more relaxed, calm, or enjoyable. It could mean a time when the weather was cooler, or when life was less stressful and more easy-going. Full definition
As soon as the weather cools this time of year, even just slightly, I get excited about sweaters, pumpkins, blankets, and warm spices. (mindbodygreen.com)
Each extra «hot» day led to a 0.4 % decrease in births, and people didn't completely make up the difference in cooler times. (sciencemag.org)
You must allow an additional 3 minutes cooling time before lifting the lid. (mybump2baby.com)