I find the best way to store dates in TM1 is by using numeric elements rather than string.
If you use numeric elements then the figure will be stored the way excel stores dates e.g. 38991
You can then format this figure so that it appears like 01/10/2006 in Excel through cell formatting and backend in TM1 through element formatting.
Storing them this way makes it a lot easier to minus or add dates to each other.
