Peredur,
Handling dates and times in MSFL is not impossible, it is just a bit "clunky" For an example to see how to add three months to a date, see the previous works from a while ago at
http://forum.equis.com/forums/permalink/17562/17081/ShowThread.aspx#17081 You should be able to achieve the similar results for time based events as the example shows for calendar based events.
Personally, I'd implement all of my time based events in external codes, simply because it is easier and much less prone to error, and faster to execute than MSFL.
Another consideration is to use the OLE capabilities to display the excel chart information in MS. Read your MS Users Manual for more informationon how to achieve this.
Hope this helps.
wabbit
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