The calculator adds or subtracts the larger time units before the smaller time units. This means that it adds years, then months, then days. Although the total number of days in January (31) and February (28) are different both are counted as 1 month. In some cases, this method can lead to unexpected results.
For example, if you add one month and one day to March 1, the result is April 2, as is to be expected (March 1 + 1 month = April 1; April 1 + 1 day = April 2).
However, if you add one month and one day to February 28, which in a common year is the day before March 1, the result is March 29, which is 4 days before April 2. The reason is that the calculator first adds 1 month to jump from February 28 to March 28, then it adds one day to arrive at March 29. If it added the days first, the result would be April 1 (February 28 + 1 day = March 1; March 1 + 1 month = April 1).
This may seem illogical at first, but the structure of the modern calendar means that there is no perfect way of calculating dates including months and years.
For calculations with several possible results, the calculator shows the Alternative result below the primary result.