Remove weekends and holidays from your table, only trading days count.

The Walk Forward formulas look right. It calculates the segments' length without the necessary bars for initializing the strategy.

Let's say you have 20 000 bars and your strategy uses 100 bars for initialization. It means that the Walk Forward tool will use 19900 bars. Let's have 5 segments and 30% OOS. The formula for calculating the segment's bars is:

segmentBars = 100 * (bars - firstBar) / ((100 - oosPercent) + segments * oosPercent);

For the above number, we will have segment bars = 100 * 19900 / ((100 - 30) + 5 * 30) = 1990000 / (70 + 150) = 9045 bars.

We will split these 9045 bars to In Sample part and OOS part as follows:

OOS bars = segmentBars * oosPercent / 100 = 9045 * 30 / 100 = 2714 bars.
IS bars = segmentBars - OOS bars = 9045 - 2714 = 6331 bars.

So we have:
Segment 1 starts at bar 100 + 0 * 2714 = 100   and ends at bar 100 + 9045 = 9145.
Segment 2 starts at bar 100 + 1 * 2714 = 2814 and ends at bar 2814 + 9045 = 11859.
...
Segment 5 starts at bar 100 + 4 * 2714 = 10956 and ends at bar 10956 + 9045 = 20001

You see everithing is fine. We have 1 bar extra because I round the calculations in this example.
Each segment walks forward with exactly OOS bars (2714).

After this math, the program takes the exact date from the corresponding bar.

Walk forward calculates the count of days as follows: bars * period / 1440

If it is M15 chart, it will be:

In Sample days = 6331 * 15 / 1440 = 66 days.
Out of Sample days = 2714 * 15 / 1440 = 28 days.

I would be glad to help if anything is not clear.

Now is better.

Each OOS period starts exactly at the following bar of the end of the previous OOS period.

The logic is the same. There are more options to set now.

I'll make further improvement of the validation next days.