Excel Import & Date Autocorrect

I have an R-based tide-prediction program that currently reads only .csv files. I need the input layer expanded so the script will also accept both .xls and .xlsx workbooks. When an Excel file is loaded,... (Budget: ₹1500 - ₹12500 INR, Jobs: Data Analysis, Data Management, Data Processing, Data Visualization, Excel, Python, Software Architecture, Visual Basic)