Please help me these extremely difficult exercises

Hello,
Hello everyone,
I have an exercise about if function in excel. Honestly, I did pretty well the previous assignments but this one is much complicated...
Please give me some instructions for this, thank you all a lot:
Task 1: Use Excel functions to check whether
the input in K16 is a date. The result is seen on L16 (the cell next to the inputted one)
The task above requires me to input a value, if this is a date, the answer is Yes. Otherwise, it is No.
It's difficult for me to help If function identify the form of inputted data being date and text .
I have tried many ways such as:
_(If K1=datevalue(1.1.1990); "Yes"; "No")
_(If K1=datevalue(dd.mm.yy); "Yes"; "No")
_(If K1=date; "Yes"; "No")
...
They are all wrong.
Task 2:
I'm looking for a formula that can count exactly the number of days according to given date and time.
For example: 2:00 AM 20/2/2015 -> 1:00 AM 21/2/2015, the excel see it as a day despite 23 hours.
Thank you all for your help.