Improve automatic checkmarks for monthly habits

Fixes #947

uhabits-core/src/commonMain/kotlin/org/isoron/platform/time/Dates.kt

@@ -72,6 +72,13 @@ data class LocalDate(val daysSince2000: Int) {
             return dayCache
         }
 
+    val monthLength: Int
+        get() = when (month) {
+            4, 6, 9, 11 -> 30
+            2 -> if (isLeapYear(year)) 29 else 28
+            else -> 31
+        }
+
     private fun updateYearMonthDayCache() {
         var currYear = 2000
         var currDay = 0

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))
                 }
             }