I recall date time filters can be a bit of a pain. First use LUKE 4.0 to check your index, for mine the "documentcreatedwhen" time field is in the format yyyymmddhhmmss, your filter only has yyyymmddhhmm, so you may need to add the seconds on there. It's doing less of an integer search and more of a text search i believe.
Try that, use Luke 4.0 to do your own search parameters on your index first to see if you can get the syntax right, then apply to the filter.
https://code.google.com/archive/p/luke/downloads