How to calculate the date diff to calculate the date diff based on a value of one column

in the fields above i would like to calculate the datediff between the trained date and the date and have a value filled into the column of Timeframe_date_diff.  ultiimately i need to use this field to determine if <1 they are pretrained and >1 trained.  When i've attempted to do this it only populates one value and one result only for the date that they were trained.  it will show post trained on that date and all others appear as pretrained before or after that dated (likely due to the trained date column being empty except for the date that they were trained on).  Not sure of how to resolve this and/or if it may be needed/possible to fill the empty cells above and/or below the trained date with the value of the date that they were trained on.

Thank you for any assistance you can provide.