Multiple values one cell and formula in another

I am trying to modify a spreadsheet that I have where I have to log changed times. To do this, it is requested that these times be in the same cell. This cell shows the original time on top of the new time via ALT+Enter. There is a second cell that has a formula to take the time from that cell and subtracts a specific ammount of time, such as 30 minutes. Obviously this no longer works with having 2 times in one cell... How would I go about making this work where I can have either a new time (not the original) or both times shown in the cell that has the formula showing the times 30 minutes earlier?

I consider myself pretty excelled at excel but this one has me stumped.

Example: Cell A1 has the formula for Cell B1-30 minutes. Cell B1 originally had one time in it and the formula in A1 worked fine, that time has been changed so now B1 has 2 times in it and cell A1 no longer shows anything.

Thanks in advance!

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