month"</span>)</div><div class='output co'>#&gt; <span class='warning'>Warning: datetime variable does not vary for 2 of the groups, no padding applied on this / these group(s)</span></div><div class='output co'>#&gt;    grp1 grp2  y       date
#&gt; 1     A    a 18 2016-06-01
#&gt; 2     A    a NA 2016-07-01
#&gt; 3     A    a NA 2016-08-01
#&gt; 4     A    a NA 2016-09-01
#&gt; 5     A    a NA 2016-10-01
#&gt; 6     A    a 10 2016-11-01
#&gt; 7     A    b 13 2016-06-01
#&gt; 8     A    b NA 2016-07-01
#&g