Fix dates before the year 2000

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

@@ -82,7 +82,10 @@ data class LocalDate(val daysSince2000: Int) {
     private fun updateYearMonthDayCache() {
         var currYear = 2000
         var currDay = 0
-
+        if (daysSince2000 < 0) {
+            currYear -= 400
+            currDay -= 146097
+        }
         while (true) {
             val currYearLength = if (isLeapYear(currYear)) 366 else 365
             if (daysSince2000 < currDay + currYearLength) {
@@ -93,10 +96,8 @@ data class LocalDate(val daysSince2000: Int) {
                 currDay += currYearLength
             }
         }
-
         var currMonth = 1
         val monthOffset = if (isLeapYear(currYear)) leapOffset else nonLeapOffset
-
         while (true) {
             if (daysSince2000 < currDay + monthOffset[currMonth]) {
                 monthCache = currMonth
@@ -105,7 +106,6 @@ data class LocalDate(val daysSince2000: Int) {
                 currMonth++
             }
         }
-
         currDay += monthOffset[currMonth - 1]
         dayCache = daysSince2000 - currDay + 1
     }

uhabits-core/src/jvmTest/java/org/isoron/platform/gui/DatesTest.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2016-2021 Álinson Santos Xavier <git@axavier.org>
+ *
+ * This file is part of Loop Habit Tracker.
+ *
+ * Loop Habit Tracker is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by the
+ * Free Software Foundation, either version 3 of the License, or (at your
+ * option) any later version.
+ *
+ * Loop Habit Tracker is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+ * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package org.isoron.platform.gui
+
+import org.isoron.platform.time.LocalDate
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class DatesTest {
+    @Test
+    fun testDatesBefore2000() {
+        val date = LocalDate(-1)
+        assertEquals(date.day, 31)
+        assertEquals(date.month, 12)
+        assertEquals(date.year, 1999)
+    }
+}