Dies non: In service terms, “dies non”
 means a day, which cannot be treated as duty for any purpose. It does 
not constitute break in service. But the period treated as ‘dies non’ 
does not qualify as service for pensionary benefits or increment.
As per the Postal Manual Volume III, 
Central Civil Services (Classification,Control and appeal) rules, 1965, 
the Absence of officials from duty without proper permission or when on 
duty in office, they have left the office without proper permission or 
while in the office, they refused to perform the duties assigned to them
 is subversive of discipline. In cases of such absence from work, the 
leave sanctioning authority may order that the days on which work is not
 performed be treated as dies non, i.e. they will 
neither count as service nor be construed as break in service. This will
 be without prejudice to any other action that the competent authorities
 might take against the persons resorting to such practices.