Change the date conditionally

Hello. I would like to locate a date when in my table the price indicated is the cheapest. But this date should not change unless the price indicated decreases further. Is there a function for that? Thanks.
In this example:
Suivi magasins.ods (86.5 KB)
If, change B6 = 2,99€ the date should be changed in D6 only but not the day after.
Formula D6: IF(AND(ismodified(B6);B6=C6);TODAY();D6)
ismodified() not exist. How make ? Thank.

You might like to look at this spreadsheet with macro for similar question by sokol92 to another recent question, Create auto update process in Calc - #6 by sokol92