Days diffs are calculated based on the beginning of the day

src/diff.js

@@ -13,16 +13,32 @@ function daysToYears(days) {
     return days * 400 / 146097;
 }
 
+function stripLocaleTime(ms) {
+    var date = new Date(ms);
+    date.setHours(0);
+    date.setMinutes(0);
+    date.setSeconds(0);
+    date.setMilliseconds(0);
+    return date.getTime();
+}
+
+function msToDay(ms) {
+    return ms / (1000 * 60 * 60 * 24);
+}
+
 export default function (from, to) {
     // Convert to ms timestamps.
     from = +from;
     to   = +to;
 
+    var fromDay = stripLocaleTime(from),
+        toDay = stripLocaleTime(to);
+
     var millisecond = round(to - from),
         second      = round(millisecond / 1000),
         minute      = round(second / 60),
         hour        = round(minute / 60),
-        day         = round(hour / 24),
+        day         = round(msToDay(toDay - fromDay)),
         week        = round(day / 7);
 
     var rawYears = daysToYears(day),

tests/index.js

@@ -157,6 +157,13 @@ describe('IntlRelativeFormat', function () {
                 expect(rf.format(future(30 * 24 * 60 * 60 * 1000))).to.equal('in 30 days');
             });
 
+            it('should output yesterday if the date is at 11:59:59pm the day and now is midnight', function () {
+                var midnight = (new Date(2017, 9, 2)).getTime();
+                var yesterday = midnight - 1;
+                var rf = new IntlRelativeFormat('en', {units: 'day'});
+                expect(rf.format(yesterday, { now: midnight })).to.equal('yesterday');
+            });
+
             it('should handle short unit formats', function () {
                 var rf = new IntlRelativeFormat('en', {units: 'minute-short'});