I need smooth drag of task on gantt so that i can save the nearest work time
If I understand you correctly you want to disable rounding the task’s start and end dates to the nearest scale marks.
Please set round_dnd_dates property to false.
Here are related article and snippet:
https://docs.dhtmlx.com/gantt/api__gantt_round_dnd_dates_config.html
http://snippet.dhtmlx.com/2ad77faad