Improve automatic checkmarks for monthly habits

Fixes #947

uhabits-core/src/jvmMain/java/org/isoron/uhabits/core/models/EntryList.kt

@@ -248,8 +248,17 @@ open class EntryList {
             for (i in num - 1 until filtered.size) {
                 val (begin, _) = filtered[i]
                 val (center, _) = filtered[i - num + 1]
-                if (begin.daysUntil(center) < den) {
-                    val end = begin.plus(den - 1)
+                var size = den
+                if (den == 30 || den == 31) {
+                    val beginDate = begin.toLocalDate()
+                    size = if (beginDate.day == beginDate.monthLength) {
+                        beginDate.plus(1).monthLength
+                    } else {
+                        beginDate.monthLength
+                    }
+                }
+                if (begin.daysUntil(center) < size) {
+                    val end = begin.plus(size - 1)
                     intervals.add(Interval(begin, center, end))
                 }
             }