Checkpoint is a package from Microsoft (available from the Microsoft R repository). Checkpoint allows users to specify an as-of date to ensure that the packages being used in a project will remain consistent, even if the user installs a newer version. Microsoft R Archive Network (MRAN) stores daily snapshots of packages and the checkpoint function in essence retrieves the snapshot corresponding to the date specified in the checkpoint function. It is used as follows:

library(checkpoint) 
checkpoint("yyyy-mm-dd") 

Further information on checkpoint is available at: https://mran.microsoft.com/documents/rro/reproducibility.