Have a look at the beautiful user manual that in provided with the application and directly reachable through F1 with contextual access.
Or in other word : RTFM

I initially read user manual for dates but In the user manual, dates are not explained clearly. could you please explain again here with good example.

Already messed up with so many dates.

what is difference b/w start date vs validated start date vs planned start date??
Similarly diff b/w planned due date vs planned end date or done date ??

It is true that one or two details are missing from the manual and the next version will be a little more explicit thanks to you.

To keep it short and simple:

Requested: customer's request
no impact on planning EXCEPT if it is the one and only constraint entered.

Validated: What you plan
Used for certain planning modes and to compare the drifts with the dates planned by the software

Planned: What the software calculates for you
You do not have direct control over these dates. The calculation takes into account absolutely all your constraints to calculate a planning with total care.

Real: What you achieve
You fill in the real work on your timesheet. It's done.

The creation date is the date you create the item
The in progress date is when you start to work on the element.
The end date is when you're done, no more left work to do.
The closing date is when you close your item.
The modification date is the last date on which the item was modified.

I don't think we can be any clearer.
Now if you have a hard time with writing, we have great training. Do not hesitate to request a quote with our contact form.