The 10 day timer starts at insertion time. There’s a magnetic switch in the sensor that triggers when the sensor is separated from the little magnet in the inserter.
This has two effects. One is that by inserting 8 hours before starting, I only have 4 hours of grace time remaining at the end of the 10 days of use. For me that doesn’t matter, because I start the new sensor in the evening, and a few hours one way or the other doesn’t matter. The other effect is that sometimes (but not always) after I start the new sensor in the Dexcom app on the phone, I see two dotted lines in the graph after the new sensor backfills the 8 hours of data it collected. When this happens I find it very helpful, because I can see how closely the new sensor was tracking with the old one. In particular, I can see if the new sensor had the typical drop to LOW, and has subsequently recovered and is tracking near the old sensor. So it gives me some indication of how much trouble to expect during the first 24 hours. But I think maybe Dexcom has been revising the app to make the overlap not show up, because the large gap between the old and new sensor lines could be embarrassing to some corporate and sales types, or maybe they are afraid the FDA will see.
Here’s an example where a new G7 was inserted around noon, warmed-up around 12:30, but only started in the app around 10pm. You can see that the first hours after warmup were reading very low compared to the good old sensor, and by 10pm the new sensor still wasn’t reading right. By the next day it was fine, but the first 24 hours for me often is rough, perhaps in part because I wear the sensor on my abdomen rather than arm. Anyway, I can see why Dexcom might not want to display graphs like this, because they “could be confusing to the patient” :eye-roll:
