dec是十二月，是英文十二月December的简写形式，正确的写法应该是“Dec.”或者“DEC.”。
一月~十二月的英文写法如下：
1、一月 JAN. Jan.=January
2、二月份 FEB. Feb.=February
3、三月份 MAR. Mar.=March
4、四月份 APR. Apr.=April
5、五月份 MAY. May=May
6、六月份 JUN. Jun.=June
7、七月份 JUL. Jul.=July
8、八月份 AUG. Aug.=August
9、九月份 SEP. Sep.=September
10、十月份 OCT. Oct.=October
11、十一月份 NOV. Nov.=November
12、十二月份 DEC. Dec.=December
拓展资料：
December
【音标】英[dɪ'sembə(r)] 美[dɪ'sembər]
【释义】the twelfth and last month of the year, between November and January
【例句】
1、The weather in early December was bleak and unpleasant.
十二月初的天气又阴冷又难受。
2、My little daughter was born in December.
我的小女儿是十二月出生的。
3、December is the last month of the year.
十二月是一年的最后一个月份。
4、Postal applications must be received by 12 December.
邮寄的申请书须于12月12日以前寄达。
5、Here it is December and New Year is coming.
现在是十二月，新年就要来了。