If you find in a sheet that a task is not appearing with other tasks in the gantt chart or the gantt chart is defaulting to a date that is far away from the rest of the project then double check that the year of all tasks are set correctly. For example, ensure that the date 12/01/17 is in 2017 and not in 1917.
You can quickly search for dates that are clearly outside the expected project timeline by filtering on a date column. For example, to look for dates that occur before your project start date, create a filter on your start date column that shows rows that are less than your project start date.
For more information about filters, see this Help Center article: https://help.smartsheet.com/articles/504659-using-filters-to-show-or-hide-sheet-data
