The same as you would use them with a calculator. There are no specific
functions built into Excel for any time series calculations, but the
SUMPRODUCT, SUMIF, COUNTIF and OFFSET functions would be the primary ones
you'd need in order to write formulas needed.
Business Analysis with Excel 2nd Edition, Que, 2002, has VBA code that
calculates and charts a time series' ACFs and PACFs. Once you've identified
the ARIMA model you're in a position to create the forecast using Excel's
worksheet functions.