This error indicates that the date formats in the file you are importing are either not matching what is in your module or they are not mapped correctly on the Time tab in the import. Let's assume that your are working with a monthly module and you are in 2013. The Anaplan module will show Jan 13, Feb 13, etc. as the months. Here are some examples of what the dates may look like in your import file and how you would set the Time tab to map them to Anaplan correctly:
Jan 13, Feb 13, Mar 13 - Match Names
1/1/13, 2/1/13, 3/1/13 - Date and choose Patterns with Separators M-D-Y
01012013 - Date and Choose Fixed-position Patterns MMDDYYYY
See more details about mapping dates here: https://community.anaplan.com/anapedia/data-integration/import#mapping-timescale.