[Plugin: Countdown Timer] Incorrect plural displayed

In Czech language we have two plurals:
2, 3, 4 ‘days’ = 2, 3, 4 ‘dny’
5 and more ‘days’ = 5 and more ‘dní’
Similar for years, months, hours, minutes, seconds.
All these plurals I set in .po and generated new
fergcorp_countdownTimer-cs_CZ.mo
Now when displaying countdown of any event there is always only one of the plurals in disregard of the number.
What more: when setting Enable JavaScript countdown to Yes then the plural ‘dní’ is displayed, whereas for No the other plural ‘dny’ is displayed.
The same pays for years, months, hours, minutes, seconds.
I am ready to supply any further information you need to solve the problem.
Thank you.