Merge pull request #27 from DumbWareio/Fix-EOM-Recurring-Event-Handling

Fixed issue #25 where recurring transactions set for the end of a month (like the 31st) weren't correctly calculating the appropriate date in shorter months.

Changes:

Improved the monthly recurring transaction logic to always use the last day of the month when the original transaction was on the last day of its month
Fixed date rollover issues that caused transactions to appear on incorrect dates (e.g., Jan 31 -> Mar 3 instead of Feb 28/29)
Added special handling for months with fewer days by using a safe approach that avoids JavaScript's automatic date adjustments
Fixed edge cases for all months regardless of their length (30/31 days)
The solution now correctly handles all cases:

Jan 31 -> Feb 28/29 -> Mar 31 -> Apr 30 -> May 31
Apr 30 -> May 31 -> Jun 30
And preserves the "last day of month" intent when appropriate

Author: abiteman

server.js

@@ -635,14 +635,64 @@ function generateRecurringInstances(transaction, startDate, endDate) {
                 currentDate.setDate(currentDate.getDate() + (pattern.interval * 7));
                 break;
             case 'month':
-                currentDate.setMonth(currentDate.getMonth() + pattern.interval);
+                // Don't modify the current date yet
+                // Check if we need to handle end-of-month special case
+                const originalDay = parseInt(transaction.date.split('-')[2]);
+                const daysInCurrentMonth = new Date(
+                    currentDate.getFullYear(), 
+                    currentDate.getMonth() + 1, 
+                    0
+                ).getDate();
+                
+                // Check if the original transaction was on the last day of its month
+                const isLastDayOfMonth = originalDay >= daysInCurrentMonth;
+                
+                // Get the number of days in the target month (after adding interval)
+                const daysInTargetMonth = new Date(
+                    currentDate.getFullYear(),
+                    currentDate.getMonth() + pattern.interval + 1,
+                    0
+                ).getDate();
+                
+                // First, create a new date for next month with a safe day value (1)
+                const nextMonth = new Date(currentDate);
+                nextMonth.setDate(1); // Set to first of month to avoid rollover
+                nextMonth.setMonth(nextMonth.getMonth() + pattern.interval);
+                
+                if (isLastDayOfMonth) {
+                    // If original was last day of month, set to last day of target month
+                    nextMonth.setDate(daysInTargetMonth);
+                } else if (originalDay > daysInTargetMonth) {
+                    // If original day doesn't exist in target month, use last day
+                    nextMonth.setDate(daysInTargetMonth);
+                } else {
+                    // Otherwise use the same day of month
+                    nextMonth.setDate(originalDay);
+                }
+                
+                // Update current date with our safely constructed date
+                currentDate = nextMonth;
                 break;
             case 'year':
                 currentDate.setFullYear(currentDate.getFullYear() + pattern.interval);
                 break;
             case 'monthday':
-                // Move to the next month, keeping the same day of month
+                // For monthday pattern, move to next month then set the day
                 currentDate.setMonth(currentDate.getMonth() + 1);
+                
+                // Get the last day of the target month
+                const lastDayOfMonth = new Date(
+                    currentDate.getFullYear(),
+                    currentDate.getMonth() + 1,
+                    0
+                ).getDate();
+                
+                // If the desired day exceeds the last day, use the last day instead
+                if (pattern.dayOfMonth > lastDayOfMonth) {
+                    currentDate.setDate(lastDayOfMonth);
+                } else {
+                    currentDate.setDate(pattern.dayOfMonth);
+                }
                 break;
         }
     }