Take a date and add days, months and years to it
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 Syntax

        GT_DateAdjust(<dDate>,<nDays>],[<nMonths>],[<nYears>]) -> dReturn

 Arguments:

      <dDate> is a date value to which the values are
      added.

      <nDays> is the number of days to add to <dDate>.

      <nMonths> is the number of months to add to <dDate>.

      <nYears> is the number of years to add to <dDate>.

 Returns:

      GT_DateAdjust() always returns a legal date.

 Description:

      Function to take a date and add days, months and
      years to it, returning the new date.

 Examples:

      // Same date next month
      dDate := GT_DateAdjust(DATE(),0,1,0)

      // Same date a year tomorrow
      dDate := GT_DateAdjust(DATE(),1,0,1)

 Source: GT_DATEA.PRG

 Author:  Martin Bryant