The maps assume a dateline of exactly 180° longitude. The actual dateline doesn't really follow that longitude completely, so those parts of the world which are on a different side of the dateline than they would be if the dateline was exactly at 180° will have to adjust the dates accordingly, This applies to some islands in the Pacific Ocean and to the easternmost parts of Russia.
The margin of error of these maps are rougly 10 pixels (on the East-West direction) on the large (2408x1208) maps, and 2 pixels on the small (602x302) maps. Although this is a lot better accuracy than other Vaishnava calendar websites, I'm not happy with this lack of accuracy. For this purpose, I'm planning to write new software, which will have very exact calculations, which will reduce the margin of error to less than 1 pixel on the big maps. This will hopefully be implemented soon.
There is a known bug, which causes some rare mahādvādaśīs to sometimes get wrongly indicated. This only affects the very rare mahādvādaśīs Jayantī, Jayā, Pāpanāśinī, and Vijayā. The other types of mahādvādaśī are correctly indicated. This will also be fixed in the new software.UPDATE 2 May 2021: This issue has been fixed.
There seems to be an issue with the extramonth. We will look into that also.