As Srini mentioned, you can use the Get first free column/row from Excel worksheet that will give you the first free row. This means that your last row is %FirstFreeRow-1%, as the first free row is the row right after the last non-empty row.
To your other question, yes, you should be able to use date and time values in comparisons. Depending on whether Excel interprets it as a valid date and time, you may either be able to do the comparison right after you read data from Excel, or (if it doesn't interpret it as a date and stores it as a text), you may need to use Convert text to date time on it first.
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