Official dexcom answer
Dexcom says this
The transmitter battery is good for up to 3 months.
But what it means is that days are counted from when you start transmitter, and force end at 90 days, even if battery still has power.
If you use dexcom phone app or dexcom receiver, answer is up to 90 days. App or receiver shows day count.
But transmitter is slowly draining power before use, either before shipment, or by user still using prior transmitter. So may not get full 90 days if battery/signal is too low before 90 days of use.
“If I start a new sensor at day 88 on the transmitter, will everything shut down exactly at day 90? And thus I only got 2 days on that otherwise new sensor? Or can I likely get 98 days on that transmitter?”
Would likely get error message and would prevent start of new sensor when less than 10 days left on transmitter day count.
Whole different story if using xDrip or Spike. But with firefly, I think still unknown if dexcom has hard stop when transmitter days get to 90. With older 80 transmitter + xDrip, definitely get more.