Whenever you are seeing a small +1 or -1 next to the time of a task, this means that they are extending into the next (+1) or previous (-1) day.
This can be really useful if you are planning something after midnight or if you are working shifts. The concept of a day, starting at 0:00 and ending at 23:59, does not fit for everyone.
When creating a task, you can set those times, by scrolling all the way up or down on the time picker. That way you can select times from 12 PM the previous day, up to 12 PM the following day.
If you selected these times accidentally, just scroll up or down in the time picker until you reach the times without a +1 or -1.