Hi @Alex , welcome to Q&A forum!
Yes, you are right, this requirement is achievable. :)
Please follow these steps:
1.Create a new view named "Days in Stage" and filter the items you want. I would suggest you use is equal to stage 1....OR is equal to stage2...OR ...... Create 7 is equal to to filter items:

2.Create a Calculated column and use this formula:
=IF(OR([Loan Stage]="Stage 1 - LOI",[Loan Stage]="Stage 7 - Closed"),Created,"N/A")
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Update1=====================
Let me explain:
What Calculated column can get is the time when this item was created: [Created], the time when it was modified [Modified], or the time in a new column. Using [Today] in a calculated column is pointless because the value get updated when the item is updated. In that way is it the same as the modified column.
I would suggest you create a Date and Time, For example we named it "CLOSED", then use this formula to calculate the number of days between the creation of the item and the closing time you specify:
=DATEDIF(Created,CLOSED,"d")

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