CGM Session Start Time (0x2AAA): byte layout
GATT characteristic · org.bluetooth.characteristic.cgm_session_start_time · read from the Bluetooth SIG’s repository on 2026-10-17 · commit 408f4a0 (2026-10-14)
CGM Session Start Time is Bluetooth GATT characteristic 0x2AAA, org.bluetooth.characteristic.cgm_session_start_time in the Bluetooth SIG's assigned numbers. The SIG's GATT Specification Supplement file lays its value out as 4 fields: Session Start Time, Time Zone, DST Offset and E2E-CRC.
- UUID
- 0x2AAA
- Identifier
org.bluetooth.characteristic.cgm_session_start_time- The SIG’s description
- The CGM Session Start Time characteristic is used to represent the time the continuous glucose monitor (CGM) session is started.The structure of this characteristic is defined below.
- Service the SIG’s files tie it to
- None: no file in the repository lists this characteristic under a service.
- Referenced by
- CGM Measurement, CGM Status, CGM Specific Ops Control Point
- Source file
- gss/org.bluetooth.characteristic.cgm_session_start_time.yaml
Byte layout: 4 fields
In the order the SIG’s structure lists them. Type and size are as published; sizes count octets except where a struct is marked as sized in bits. The offset is printed only where every earlier field has a fixed size; where an earlier field can be absent or vary, the cell names what it depends on. “Present when” is read from the field’s own “Present if” line, shown in full in the field notes below.
| Offset | Field | Type | Size | Present when | Unit |
|---|---|---|---|---|---|
| 0 | Session Start Time | struct | 7 | Always (fixed size) | — |
| 7 | Time Zone | uint8 | 1 | Always (fixed size) | — |
| 8 | DST Offset | uint8 | 1 | Always (fixed size) | — |
| 9 | E2E-CRC | uint16 | 0 or 2 | Present if E2E-CRC Supported bit in CGM Feature characteristic is set to 1 | — |
Field notes
Each field’s text in the SIG’s file, line by line.
- Session Start Time struct · 7
- Refer to the Date Time characteristic in Date Time (0x2A08)
- Time Zone uint8 · 1
- Refer to the Time Zone characteristic in Time Zone (0x2A0E)
- DST Offset uint8 · 1
- Refer to the DST Offset characteristic in DST Offset (0x2A0D)
- E2E-CRC uint16 · 0 or 2
- CRC calculated over all fields. See
§ crc_calculationfor details.Present if E2E-CRC Supported bit in CGM Feature characteristic is set to 1
Guides on this site
No guide on this site names 0x2AAA yet. The connected-devices guides cover pairing, subscribing and testing for the device types they do name.
Questions
- What is the UUID of the CGM Session Start Time characteristic?
- 0x2AAA. The Bluetooth SIG's characteristic_uuids.yaml lists CGM Session Start Time as 0x2AAA with the identifier org.bluetooth.characteristic.cgm_session_start_time. No file in the SIG's repository ties it to a service: the repository lists service and characteristic UUIDs separately. CGM Measurement, CGM Status and CGM Specific Ops Control Point refer to it in their own structure.
- Which CGM Session Start Time fields are optional?
- 1 of its 4 fields has a published size that can be zero: E2E-CRC (0 or 2; Present if E2E-CRC Supported bit in CGM Feature characteristic is set to 1). Session Start Time (7), Time Zone (1) and DST Offset (1) have a fixed size.
- In what order does the CGM Session Start Time structure list its fields?
- 1. Session Start Time (struct, 7, offset 0); 2. Time Zone (uint8, 1, offset 7); 3. DST Offset (uint8, 1, offset 8); 4. E2E-CRC (uint16, 0 or 2, offset 9). Sizes are as the SIG publishes them; an offset is given only where every earlier size is a plain number.
Source
Read on 2026-10-17 from gss/org.bluetooth.characteristic.cgm_session_start_time.yaml and assigned_numbers/uuids/characteristic_uuids.yaml in the Bluetooth SIG’s public repository (branch main, commit 408f4a0, committed 2026-10-14), by scripts/fetch-bluetooth-gatt.mjs. The file’s notice says it is proprietary to Bluetooth SIG, that furnishing it grants no licence, and that it is subject to change without notice; it is © Bluetooth SIG, Inc., and its text is quoted here for identification. Generated by AIFitnessAPI, which is not affiliated with Bluetooth SIG; the SIG’s file remains the authority. The full notice and the conventions used on these pages are on the reference hub.