AR6 WGI adopts a common set of reference years and time periods to assess observed and projected climate change, namely the pre- industrial period, the current 'modern' period and future reference time periods. The IPCC Glossary  defines the pre-industrial period as 'the multi-century period prior to the onset of large-scale industrial activity around 1750. The reference period 1850-1900 is used to approximate pre-industrial global mean surface temperature .' The 'modern' period is defined as 1995 to 2014 in AR6, while three future reference periods are used for presenting climate change projections, namely near term , mid-term  and long term , in both the AR6 WGI and WGII reports. Importantly, the historical rate of warming assessed by WGI in AR6 is different to that assessed in AR5 and Special Report on Global Warming of 1.5℃ , due to methodological updates ). This means that the 'modern' period is assessed as slightly warmer compared to 1850-1900 than it would have been with AR5-era methods. This also has implications for the projected timing of reaching policy-relevant levels of global warming, which need to be understood.
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report