I love Excel and use it daily. Working with dates can frustrating if you don't know the keyboard shortcuts and functions/formulas.
=NOW() and =Today() insert the current time/date and update whenever the sheet updates. This is good if you have a report and you want to show the date when it was last updated.
=DAY(TODAY()) is the current day of the month.
=MONTH(TODAY()) is the current month.
If you want the dates to stay put, use CTRL and ; (semi-colon). The time is CTRL + SHIFT + ;.