how to fill a range of cells by a date-type serie?

[barewillysprl <barewillysprl scarlet be>]

how to fill a range of cells by a date-type serie?

Hi,
Here on gnumeric-1.10.17

The gnumeric manual tells to define date serie the same way as number.

I tried reproduce that process using the Ctrl key on a range of cells 
whose first cell has been filled with Jan; but nope :(

I also tried using the Menu/Data/Fill/Series and there checking the 
boxes
Thumbnail Series
Series in: Row (or Column)
Type: date
Start value: Jan
Step value: her have tried various values as 1, m, mm, etc but did not 
succeed ! :(
Stop value:Dec

Thumbnail Options
Date Unith: Month

Thumbnail Output
Output range: Sheet1!A1:A12

Many thanks for any solution especially for filling the «Step value:»

Best regards

Charlie