Empty date cells shows as 01/01/1900, but I want them left blank

Hi!

I'm a beginner, so please be patient! I have created a simple spreadsheet with names and dates:

F3 is the formatted to dd/mm/yy

• H3 has formula =F3+7 (1 week), J3 has the formula =F3+14 (2 weeks)
• If F3 is empty the following is shown in H3 and J3 07/01/00 and 14/01/00 respectively, but I want these cells to be blank when F3 is empty.
• Then, when someone enters a date in F3, it provides calculated dates for H3 & J3

I then want to add some conditional formatting so that these highlight red when they are due/overdue.  Thinking of making the text white then (highlight cell rules > less than todays date)

This is driving me mad!  I've looked at other similar questions, but they all seem too complex.

Any help is greatly appreciated!