In one sentence
A single day with a sensor is a weather report; a month gives every meal, every bad night, every hard session and, for women, the whole cycle their turn, and that is the baseline VT measures the next month against.
Why fourteen days is the floor
The international consensus on reading sensor data recommends at least 14 days of recording with the sensor active for at least 70 percent of that time before the summary figures are taken as representative of a person (Battelino 2019; Danne 2017), because fewer days over-weight whichever days happened to be unusual. VT applies the same floor to the glucose management indicator and to the month's summaries: with fewer days or less coverage, the figure is shown as provisional.
Why VT asks for twenty-eight
Two sensors, back to back. The second fortnight catches what the first missed: the weekend that was different from the week, the second half of the menstrual cycle, the week with the cold, the meals you had not got round to logging. It also gives the meal library enough repeats to mean something, since a meal needs to be eaten on more than one day before its response can be trusted (see Reading a CGM without fooling yourself, point 14). The Baseline Month report on day 28 is the reference for everything that follows: the daily mean, time in range, how often you rose and how far, your meal library, and an estimated HbA1c, all against which the next month and the next year are compared. The Vitality Score's glucose area does not wait for it: it reads your sensor's daily average from the first full day.
What to do during it
Log meals, as far as you can, because the meal library is the point; log alcohol, training, sleep and sick days, because they are what explains the line; keep the sensor on the same arm for both sensors if you can, so the two fortnights compare. Nothing else: the month is for looking, not for changing things yet.
How Vitality Tracker uses it
The Baseline Month's summaries are frozen as a reference the day it ends and shown beside every later period. A later month can be marked as a new baseline when something has changed for good.
Sources
- Battelino T, et al. Clinical targets for continuous glucose monitoring data interpretation: recommendations from the International Consensus on Time in Range. Diabetes Care 2019;42:1593–603. doi:10.2337/dci19-0028
- Danne T, et al. International consensus on use of continuous glucose monitoring. Diabetes Care 2017;40:1631–40. doi:10.2337/dc17-1600
- Vitality Tracker, CGM page brief (19 October 2026).