Symbol to use to interpret "Earliest" or "Latest" in StartDate or EndDate
Notes
By default Earliest means the oldest date and Latest the newest date of any symbol in the current data file.
If DateSym is specified then that symbol is used instead.
A good use of DateSym is to avoid the common problem where adding a currency symbol like AUDUSD to your data file causes the "latest" bar to be one for which no stocks have bars.
Do not prefix the symbol with $ -- that is only required for symbol references within formulas.
