It's not possible to set a target date so Fitbit could automatically determine which the daily deficit will be until reaching the goal.
When the deficit is to small (let's say having a deficit of 100 calories a day), it's possibly to eat on maintenance instead even when weighing foods:
* The actual calories eaten can be more or less than displayed on a package;
* Having eaten a bit more than actually logged;
..
Currently the target date is based on current weight - target weight (and which deficit plan is chosen).
It's possible to stay at the same weight but being leaner (having a smaller tummy, being more toned).. when body fat % dropped, but by exercising, getting more muscle mass.
Aim to change body composition, rather than focusing on weight alone. There can also be weight loss if holding a few pounds food remnants in the tummy for a longer time (being constipated), then the next day weighing suddenly a few pounds less when all food remnants left the body.
There can also be water upkeep if eaten to much salt, or when it's warmer outside & not drinking enough so your body holds onto any water it can to survive.
Aside weighing, a measurement tape can be used to see progress & the mirror.
Feel free to add your suggestions to the 'Feature Requests' Sub Forum, where users can vote, add comments,.. if there's a certain interest then there's a chance the developers pick it up and it'll be developed.
Tip: Search if the idea already has been suggested and add your vote / comment! 
Feature Suggestions
URL: Click to go to the Feature Requests Forum
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