Fix differ.

my-bois/src/helpers/diff.js

@@ -3,6 +3,10 @@ export function diffString(diff) {
 }
 
 export default function calculateDiff({program, cooldownMin = null, minutes, seconds, calories}) {
+  if (cooldownMin < 0) {
+    cooldownMin = null
+  }
+
   const {warmupMin, warmupCpm, cpm, cooldownCpm} = program;
   const actualWarmup = cooldownMin !== null && cooldownMin > 0
     ? Math.min(warmupMin, cooldownMin)