How to split a dataframe into smaller constituent dataframes

I have read a large dataset into a dataframe using RODBC, the rows of data 
in the dataframe are (integer) timestamped and I would like to divide the 
original dataframe into n smaller dataframes where dataframe 1 contains all 
rows that had timestamps falling in the period 0-x1 minutes, dataframe 2 
contains all rows that had timestamps falling between x1+1 and x2, etc.. 
Does anyone know how to do this?


Thanks

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