Here's Siglent's policy statement on the matter with regard to the SDS2104X+ I'd purchased just over 3 years ago.
2023 Calibration statement:
Recommended Due Date for Calibration
SIGLENT has determined that the factory calibration of the following models is not significantly affected by storage of
up to 18 months before first-time use. Cal Interval should start at the time the unit is placed in service or 18 Months past the “Date of Calibration” on the certificate received with the unit.
Models effected by this notice:
·
SDS1000DL+ Series Oscilloscopes
·
SDS1000X-E/U Series Oscilloscopes
·
SDS2000X-E Series Oscilloscopes
·
SSA3000X Series Spectrum Analyzers
·
SPS5000X Power Supplies
·
SPD1000X Power Supplies
SPD3000 Power Supplies
SDG1000X arbitrary waveform generators
SDG2000X arbitrary waveform generators
Determine the date when the next Cal is due (refer to the examples shown below)
Example 1: First-time use of instrument is less than 18 Months after “Date of Calibration”.
Date of Calibration: 2019-06-17
Example calibration interval: 12 months
First-time use of instrument: 2019-8-17
First-time use + Example calibration interval = Date for next calibration
2019-8-17 + 12 months = 2020-8-17
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Example 2: First-time use of instrument is more than 18 Months after “Date of Calibration”.
Date of Calibration: 2019-06-17
Example calibration interval: 12 months
First-time use of instrument: 2021-1-17
Date of Calibration + 18 Months + Example calibration interval = Date for next calibration
2019-6-17 + 18 Months + 12 months = 2021-12-17
https://siglentna.com/service-and-support/calibration-certificate/