This is a sample of what you see in your xls dataset that you download from KoBoToolbox:
As seen in the image above, it is a summary of the international standard date and time notation following the International Standard ISO 8601 notation. The front part before the T is the date while the latter part is the time when the filled up form was recorded to the server.
According to the International Standard ISO 8601 notation, if a date and a time are displayed on the same line, then the date should be written in front of the time. If a date and a time value are stored together in a single data field, they should be separated by a latin capital letter T.