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To make a comparison between dates present in various databases using R, you can follow these steps:

1. Connect to the databases containing the date columns using the appropriate R packages and connection methods (e.g., RMySQL for MySQL databases or RODBC for ODBC databases).

2. Load the date columns as R data frames using the dbReadTable() function.

3. Convert the date columns to the Date data type in R using the as.Date() function.

4. Use the merge() or join() functions to combine the data frames based on a common column, usually a unique identifier or date.

5. Apply any necessary transformations or calculations on the combined data frame to compare the dates, such as calculating the time difference between two dates or determining the earliest/latest date in a dataset.

6. Output the results of the comparison as a new data frame or table in the database, or save it as a CSV file for further analysis or visualization.