There is a Libre clever monitoring sensor that sticks on your upper arm to monitor glucose in the interstitial fluid. Quite discreet and painless to affix - it's only when you remove it that you notice the size of the vampire tap probe. The sensor unit has NFC communication for an occasional "flash" scan from either a dedicated Reader or a smartphone app. The sensor holds 8 hours of effectively continuous glucose readings. The NFC allows them to be uploaded to the reader/smartphone - hopefully before the sensor buffer cycles. The values are stored in the Reader for 90 days with their correct wall-clock time stamp reported in the Reader's graphs. Any sensor buffer overrun gap is shown at the correct time period for which it lost buffered values.
As the probe is an electo-chemical sensor it has a pre-defined period of active use of 14 days. On the 14th day you get a regular count-down to the time when it will stop monitoring.
It gets interesting if you use both a Reader and a smartphone for scans. The Reader activates the sensor initially - and the smartphone then gets associated with the sensor afterwards.
This week the Reader interrogated the sensor - and was correctly indicating about 30 minutes left. However - the smartphone app said it had already expired. Scratching my head on that - both the Reader and smartphone wall-time clocks appear to be in reasonable UK DST sync.