Assuming you use JMP, you need to use a continuous, date-formatted column as x-variable.
One way to convert your date strings into date-values is to use a column formula. In the jsl example below the strings are found in column "x" and the formula in column "Date" turn "x" into the corresponding date (here based on the first day of the measurement interval).
dt = New Table( "example",
Add Rows( 4 ),
New Column( "x",
Character,
Nominal,
Set Values( {"January 9, 10, 11", "February 11, 12, 13", "April 11, 12", "July 9, 10"} )
)
);
dt << New Column( "Date",
Numeric,
Continuous,
Format( "ddMonyyyy", 12 ),
Input Format( "ddMonyyyy" ),
formula( Parse Date( Words( :x, ", " )[2] || Left( :x, 3 ) || "2013", "ddMonyyyy" ) )
);