Dates are by default treated as numbers. Using dates for x-values in a “X-Y (Scatter)” diagram requires the numerical representation.
Calendaric dates are represented by the integer number of days since 1899-12-30 which was defined to be day number zero (an error originally). (There were a few outdated alternatives.)
To apply an extrapolation formula you won’t need to do anything about this. The formula will accept the numeric value of any cell independent on the format used for display in this cell.
Your example of application is bringing up two issues, however:
a) The reliance of the y-values on the x-values is clearly not linear (or of another well known functional type).
b) The coefficients taken from the chart’s trend-line equation cannot be used as variable paremeters. They must be typed in.
Te second issue can be solved with the help of LINEST (or LOGEST under different conditions). There is also the TREND function.
(By the way: Don’t use the USA date format in international communication. To avoid dangerous misinterpretations there is the YYYY-MM-DD format in compliance with ISO 8601.)