First of all, a tester should know, Which type of Format is using.
Eg: DD/MM/YY or DD/MM/YYYY or MM/DD/YYYY or DD/Jan/yyyy Like so on many...........
For eg: 01-Jan-2007.
Date: Date may be like x/xx ------- it may be 01 or 1.
Month: 1)It may be like Jan --- it should be 3 words, Beginning should be Capital, remaining should be small.
2)It may be like January ------ The beginning should be capital or not.
3) It may be numbers.... like 01 0r 1
Year: It may be yy or yyyy ----- for eg take 2007 .. we can give 07 or 2007. After 9999 th year what will happen, if we give 20071 What will happen. A tester should test all this Scanarios..... and many more comes to the picture.