I search informations about this display and Arduino development board and I find few articles, but more usefull was article from http://www.14core.com/wiring-the-6-digit-7-segment-2-4-inch-ht1621-lcd-display-module-on-microcontroller/. I test the schematic presented in article
with HT1621_test.ino sketch and on display is 23:45 clock with animated second and after 17 degree Celsius:
This mode can be see it on HT1621 6 digit 7 segment LCD display test video
and random temperature (negative and positive)
In HT1621 6 digit 7 segment LCD display - test 2 video you can see this display mode: