I suggest filing dates as YYYY MM-DD. 
If, like me, you use Microsoft Office tools such as Word and Excel to keep track of your genealogy info, the software does not always keep track of dates as dates but rather as text. Sorting dates in the format above will result in a continuous timeline when using Office’s internal “Sort” tool.
1900 12-30
1901 01-
1902
Sometimes you know a year without a month, or a month but no date. Using a different divider between month and date allows you to do searches based on year and month:
Looking for a record in January 1900? Search 1901 01-
Looking for a record of December 30 but don’t remember the year? Search 12-30