Hi @Wagemans_Frank,
To be honest, I don't get you.
Sorry that I could not reproduce this issue in my environment.
To do the calculation, we need to convert the data into number value, not sure how you format your date string in the Text column, however it is another workload to convert the text date string into date value, which costs a lot of time, I don't think this makes sense.
To be more efficient, you should create Date column saving date value and number column to save the days, and in Power Apps do the calculation thing, using the following formula to calculate the end date:
DateAdd(ThisItem.START, ThisItem.Days, Days)
Then Modify the formula as below:
If(Now() >= ThisItem.START&&Now() <= DateAdd(ThisItem.START, ThisItem.Days, Days),Color.Green,Color.Red)