Fix GetPreviousOccurrence returning a day in a disallowed month

When the time-of-day search rolls back past the 1st of the month, dayLimit
(and maxDay) can drop below the first valid day of month. The day-of-month
'*' fast path in TryGetPreviousDay returned actualDay = 0, and
DateTimeToTicks(year, month, 0, ...) underflowed into the last day of the
previous month, which is not re-validated against the month field. Guard
against maxDay below the first day of month so the caller advances to the
previous allowed month instead.

src/Cronos/CronExpression.cs

@@ -995,6 +995,15 @@ namespace Cronos
             var lastDayOfMonth = CalendarHelper.GetDaysInMonth(year, month);
             var maxDay = dayLimit > lastDayOfMonth ? lastDayOfMonth : dayLimit;
 
+            // Rolling back past the 1st can drop maxDay below the first valid day,
+            // and the "*" fast path would then underflow into the previous month.
+            if (maxDay < CronField.DaysOfMonth.First)
+            {
+                day = default;
+                actualDay = default;
+                return false;
+            }
+
             if (HasFlag(CronExpressionFlag.NearestWeekday))
             {
                 day = HasFlag(CronExpressionFlag.DayOfMonthLast)

tests/Cronos.Tests/CronExpressionReverseFacts.cs

@@ -125,6 +125,27 @@ namespace Cronos.Tests
             Assert.Equal(GetUtcDateTime(expectedString), previous);
         }
 
+        [Theory]
+
+        // Rolling back past midnight of the 1st must skip to the previous allowed
+        // month, not land in the preceding disallowed one.
+        [InlineData("0 17 * 7 *", "2017-07-01 14:00:00", "2016-07-31 17:00:00")]
+        [InlineData("37-57/3 17-21 * 7 *", "2017-07-01 14:20:00", "2016-07-31 21:55:00")]
+        [InlineData("0 0 * 3 *", "2020-03-01 00:00:00", "2019-03-31 00:00:00")]
+
+        // Control: an unrestricted month field must still roll back normally.
+        [InlineData("0 17 * * *", "2017-07-01 14:00:00", "2017-06-30 17:00:00")]
+        public void GetPreviousOccurrence_DoesNotReturnDayInDisallowedMonth_WhenTimeRollsBackPastFirstOfMonth(
+            string cronExpression, string fromString, string expectedString)
+        {
+            var expression = CronExpression.Parse(cronExpression, CronFormat.Standard);
+            var from = GetUtcDateTime(fromString);
+
+            var previous = expression.GetPreviousOccurrence(from, inclusive: false);
+
+            Assert.Equal(GetUtcDateTime(expectedString), previous);
+        }
+
         [Fact]
         public void GetPreviousOccurrence_ReturnsCorrectDate_WhenMacroExpressionHasJitterSeed()
         {