As the Christmas event in Terraria is drawing to a close on the 31st, I have a question. Since I will probably not be in hard mode and have defeated the frost legion by the time that Christmas is over, can I alter my device's date and time back to "Christmas time" and still be able to receive presents, fight the frost legion, and use all of the other Christmas features?
Yes, you can manually set your device's date to get any of the season events to occur. This is mentioned on the wiki page for the Christmas event:
Hint: You can "activate" Christmas by setting your computer's or mobile's date to a day between December 15th and 31st.
The Seasonal event page lists all of the dates during which the various events are active.