This niffty tool locates letters of similar date. See an example on the right with several blue lines under the 6 May 1881. On the first row, the blue line (it'll turn red when you mouse over) covers the day, month and year and so by clicking it, letters of the exact same date(day,month and year) will be displayed. On the third row, the blue line is split in two. To the left it covers the day and month and so locates letters of the same calendar day. To the right it covers just the year and so will find letters of the same year. Using this tool, one can display letters by a century, a decade, a month, a year, a calendar day, a month of a year, a day and an exact date. Play with it for a minute and everything will become self explanatory.
The date tag allows one to search for a letter with specific dates. One can search for a specific date ( day month and year) or a calendar date (day and month) or any century,decade or year. 'x' is the wild character and organised as YYYYMMDD so letters from december 1944 would be 194412xx and letters from the 1860s would be 186xxxxx