To convert a date, read across to the third column for the full date with all the abbreviations the Romans would have used on inscriptions. The initial 'A.D.' stands for ante diem - days before - not AD Anno Domini. It can be omitted.

Modern	Roman	Full date
January	IANVARIVS	IAN.
1	KALENDS	KAL. IAN
2	IV	A.D. IV NON. IAN.
3	III	A.D. III NON. IAN.
4	PRIDIE	PRID. NON. IAN.
5	NONES	NON. IAN.
6	VIII	A.D. VIII ID. IAN.
7	VII	A.D. VII ID. IAN.
8	VI	A.D. VI ID. IAN.
9	V	A.D. V ID. IAN.
10	IV	A.D. IV ID. IAN.
11	III	A.D. III ID. IAN.
12	PRIDIE	PRID. ID. IAN.
13	IDES	ID. IAN.
14	XIX	A.D. XIX KAL. FEB.
15	XVIII	A.D. XVIII KAL. FEB.
16	XVII	A.D. XVII KAL. FEB.
17	XVI	A.D. XVI KAL. FEB.
18	XV	A.D. XV KAL. FEB.
19	XIV	A.D. XIV KAL. FEB.
20	XIII	A.D. XIII KAL. FEB.
21	XII	A.D. XII KAL. FEB.
22	XI	A.D. XI KAL. FEB.
23	X	A.D. X KAL. FEB.
24	IX	A.D. IX KAL. FEB.
25	VIII	A.D. VIII KAL. FEB.
26	VII	A.D. VII KAL. FEB.
27	VI	A.D. VI KAL. FEB.
28	V	A.D. V KAL. FEB.
29	IV	A.D. IV KAL. FEB.
30	III	A.D. III KAL. FEB.
31	PRIDIE	PRID. KAL. FEB.
February	FEBRVARIS	FEB.
Non-leap year
1	KALENDS	KAL. FEB.
2	IV	A.D. IV NON. FEB.
3	III	A.D. III NON. FEB.
4	PRIDIE	PRID. NON. FEB.
5	NONES	NON. FEB.
6	VIII	A.D. VIII ID. FEB.
7	VII	A.D. VII ID. FEB.
8	VI	A.D. VI ID. FEB.
9	V	A.D. V ID. FEB.
10	IV	A.D. IV ID. FEB.
11	III	A.D. III ID. FEB.
12	PRIDIE	PRID. ID. FEB.
13	IDES	ID. FEB.
14	XVI	A.D. XVI KAL. MART.
15	XV	A.D. XV KAL. MART.
16	XIV	A.D. XIV KAL. MART.
17	XIII	A.D. XIII KAL. MART.
18	XII	A.D. XII KAL. MART.
19	XI	A.D. XI KAL. MART.
20	X	A.D. X KAL. MART.
21	IX	A.D. IX KAL. MART.
22	VIII	A.D. VIII KAL. MART.
23	VII	A.D. VII KAL. MART.
24	VI	A.D. VI KAL. MART.
25	V	A.D. V KAL. MART.
26	IV	A.D. IV KAL. MART.
27	III	A.D. III KAL. MART.
28	PRIDIE	PRID. KAL. MART.
Leap year
23	VII	A.D. VII KAL. MART.
24	VI	A.D. VI KAL. MART.
25	BIS VI	A.D. BIS VI KAL. M.
26	V	A.D. V KAL. MART.
27	IV	A.D. IV KAL. MART.
28	III	A.D. III KAL. MART.
29	PRIDIE	PRID. KAL. MART.
March	MARTIVS	MART.
1	KALENDS	KAL. MART.
2	VI	A.D. VI NON. MART.
3	V	A.D. V NON. MART.
4	IV	A.D. IV NON. MART.
5	III	A.D. III NON. MART.
6	PRIDIE	PRID. MART.
7	NONES	NON. MART.
8	VIII	A.D. VIII ID. MART.
9	VII	A.D. VII ID. MART.
10	VI	A.D. VI ID. MART.
11	V	A.D. V ID. MART.
12	IV	A.D. IV ID. MART.
13	III	A.D. III ID. MART.
14	PRIDIE	PRID. ID. MART.
15	IDES	ID. MART.
16	XVII	A.D. XVII KAL. APR.
17	XVI	A.D. XVI KAL. APR.
18	XV	A.D. XV KAL. APR.
19	XIV	A.D. XIV KAL. APR.
20	XIII	A.D. XIII KAL. APR.
21	XII	A.D. XII KAL. APR.
22	XI	A.D. XI KAL. APR.
23	X	A.D. X KAL. APR.
24	IX	A.D. IX KAL. APR.
25	VIII	A.D. VIII KAL. APR.
26	VII	A.D. VII KAL. APR.
27	VI	A.D. VI KAL. APR.
28	V	A.D. V KAL. APR.
29	IV	A.D. IV KAL. APR.
30	III	A.D. III KAL. APR.
31	PRIDIE	PRID. KAL. APR.
April	APRILIS	APR.
1	KALENDS	KAL. APR.
2	IV	A.D. IV NON. APR.
3	III	A.D. III NON. APR.
4	PRIDIE	PRID. NON. APR.
5	NONES	NON. APR.
6	VIII	A.D. VIII ID. APR.
7	VII	A.D. VII ID. APR.
8	VI	A.D. VI ID. APR.
9	V	A.D. V ID. APR.
10	IV	A.D. IV ID. APR.
11	III	A.D. III ID. APR.
12	PRIDIE	PRID. ID. APR.
13	IDES	ID. APR.
14	XVIII	A.D. XVIII KAL. MAI.
15	XVII	A.D. XVII KAL. MAI.
16	XVI	A.D. XVI KAL. MAI.
17	XV	A.D. XV KAL. MAI.
18	XIV	A.D. XIV KAL. MAI.
19	XIII	A.D. XIII KAL. MAI.
20	XII	A.D. XII KAL. MAI.
21	XI	A.D. XI KAL. MAI.
22	X	A.D. X KAL. MAI.
23	IX	A.D. IX KAL. MAI.
24	VIII	A.D. VIII KAL. MAI.
25	VII	A.D. VII KAL. MAI.
26	VI	A.D. VI KAL. MAI.
27	V	A.D. V KAL. MAI.
28	IV	A.D. IV KAL. MAI.
29	III	A.D. III KAL. MAI.
30	PRIDIE	PRID. KAL. MAI.
May	MAIVS	MAI.
1	KALENDS	KAL. MAI.
2	VI	A.D. VI NON. MAI.
3	V	A.D. V NON. MAI.
4	IV	A.D. IV NON. MAI.
5	III	A.D. III NON. MAI.
6	PRIDIE	PRID. MAI.
7	NONES	NON. MAI.
8	VIII	A.D. VIII ID. MAI.
9	VII	A.D. VII ID. MAI.
10	VI	A.D. VI ID. MAI.
11	V	A.D. V ID. MAI.
12	IV	A.D. IV ID. MAI.
13	III	A.D. III ID. MAI.
14	PRIDIE	PRID. ID. MAI.
15	IDES	ID. MAI.
16	XVII	A.D. XVII KAL. IVN.
17	XVI	A.D. XVI KAL. IVN.
18	XV	A.D. XV KAL. IVN.
19	XIV	A.D. XIV KAL. IVN.
20	XIII	A.D. XIII KAL. IVN.
21	XII	A.D. XII KAL. IVN.
22	XI	A.D. XI KAL. IVN.
23	X	A.D. X KAL. IVN.
24	IX	A.D. IX KAL. IVN.
25	VIII	A.D. VIII KAL. IVN.
26	VII	A.D. VII KAL. IVN.
27	VI	A.D. VI KAL. IVN.
28	V	A.D. V KAL. IVN.
29	IV	A.D. IV KAL. IVN.
30	III	A.D. III KAL. IVN.
31	PRIDIE	PRID. KAL. IVN.
June	IVNIVS	IVN.
1	KALENDS	KAL. IVN.
2	IV	A.D. IV NON. IVN.
3	III	A.D. III NON. IVN.
4	PRIDIE	PRID. NON. IVN.
5	NONES	NON. IVN.
6	VIII	A.D. VIII ID. IVN.
7	VII	A.D. VII ID. IVN.
8	VI	A.D. VI ID. IVN.
9	V	A.D. V ID. IVN.
10	IV	A.D. IV ID. IVN.
11	III	A.D. III ID. IVN.
12	PRIDIE	PRID. ID. IVN.
13	IDES	ID. IVN.
14	XVIII	A.D. XVIII KAL. IVL.
15	XVII	A.D. XVII KAL. IVL.
16	XVI	A.D. XVI KAL. IVL.
17	XV	A.D. XV KAL. IVL.
18	XIV	A.D. XIV KAL. IVL.
19	XIII	A.D. XIII KAL. IVL.
20	XII	A.D. XII KAL. IVL.
21	XI	A.D. XI KAL. IVL.
22	X	A.D. X KAL. IVL.
23	IX	A.D. IX KAL. IVL.
24	VIII	A.D. VIII KAL. IVL.
25	VII	A.D. VII KAL. IVL.
26	VI	A.D. VI KAL. IVL.
27	V	A.D. V KAL. IVL.
28	IV	A.D. IV KAL. IVL.
29	III	A.D. III KAL. IVL.
30	PRIDIE	PRID. KAL. IVL.
July	IVLIVS	IVL.
1	KALENDS	KAL. IVL.
2	VI	A.D. VI NON. IVL.
3	V	A.D. V NON. IVL.
4	IV	A.D. IV NON. IVL.
5	III	A.D. III NON. IVL.
6	PRIDIE	PRID. IVL.
7	NONES	NON. IVL.
8	VIII	A.D. VIII ID. IVL.
9	VII	A.D. VII ID. IVL.
10	VI	A.D. VI ID. IVL.
11	V	A.D. V ID. IVL.
12	IV	A.D. IV ID. IVL.
13	III	A.D. III ID. IVL.
14	PRIDIE	PRID. ID. IVL.
15	IDES	ID. IVL.
16	XVII	A.D. XVII KAL. AVG.
17	XVI	A.D. XVI KAL. AVG.
18	XV	A.D. XV KAL. AVG.
19	XIV	A.D. XIV KAL. AVG.
20	XIII	A.D. XIII KAL. AVG.
21	XII	A.D. XII KAL. AVG.
22	XI	A.D. XI KAL. AVG.
23	X	A.D. X KAL. AVG.
24	IX	A.D. IX KAL. AVG.
25	VIII	A.D. VIII KAL. AVG.
26	VII	A.D. VII KAL. AVG.
27	VI	A.D. VI KAL. AVG.
28	V	A.D. V KAL. AVG.
29	IV	A.D. IV KAL. AVG.
30	III	A.D. III KAL. AVG.
31	PRIDIE	PRID. KAL. AVG.
August	AVGVSTVS	AVG.
1	KALENDS	KAL. IAN
2	IV	A.D. IV NON. AVG.
3	III	A.D. III NON. AVG.
4	PRIDIE	PRID. NON. AVG.
5	NONES	NON. AVG.
6	VIII	A.D. VIII ID. AVG.
7	VII	A.D. VII ID. AVG.
8	VI	A.D. VI ID. AVG.
9	V	A.D. V ID. AVG.
10	IV	A.D. IV ID. AVG.
11	III	A.D. III ID. AVG.
12	PRIDIE	PRID. ID. AVG.
13	IDES	ID. AVG.
14	XIX	A.D. XIX KAL. SEPT.
15	XVIII	A.D. XVIII KAL. SEPT.
16	XVII	A.D. XVII KAL. SEPT.
17	XVI	A.D. XVI KAL. SEPT.
18	XV	A.D. XV KAL. SEPT.
19	XIV	A.D. XIV KAL. SEPT.
20	XIII	A.D. XIII KAL. SEPT.
21	XII	A.D. XII KAL. SEPT.
22	XI	A.D. XI KAL. SEPT.
23	X	A.D. X KAL. SEPT.
24	IX	A.D. IX KAL. SEPT.
25	VIII	A.D. VIII KAL. SEPT.
26	VII	A.D. VII KAL. SEPT.
27	VI	A.D. VI KAL. SEPT.
28	V	A.D. V KAL. SEPT.
29	IV	A.D. IV KAL. SEPT.
30	III	A.D. III KAL. SEPT.
31	PRIDIE	PRID. KAL. SEPT.
September	SEPTEMBER	SEPT.
1	KALENDS	KAL. SEPT.
2	IV	A.D. IV NON. SEPT.
3	III	A.D. III NON. SEPT.
4	PRIDIE	PRID. NON. SEPT.
5	NONES	NON. SEPT.
6	VIII	A.D. VIII ID. SEPT.
7	VII	A.D. VII ID. SEPT.
8	VI	A.D. VI ID. SEPT.
9	V	A.D. V ID. SEPT.
10	IV	A.D. IV ID. SEPT.
11	III	A.D. III ID. SEPT.
12	PRIDIE	PRID. ID. SEPT.
13	IDES	ID. SEPT.
14	XVIII	A.D. XVIII KAL. OCT.
15	XVII	A.D. XVII KAL. OCT.
16	XVI	A.D. XVI KAL. OCT.
17	XV	A.D. XV KAL. OCT.
18	XIV	A.D. XIV KAL. OCT.
19	XIII	A.D. XIII KAL. OCT.
20	XII	A.D. XII KAL. OCT.
21	XI	A.D. XI KAL. OCT.
22	X	A.D. X KAL. OCT.
23	IX	A.D. IX KAL. OCT.
24	VIII	A.D. VIII KAL. OCT.
25	VII	A.D. VII KAL. OCT.
26	VI	A.D. VI KAL. OCT.
27	V	A.D. V KAL. OCT.
28	IV	A.D. IV KAL. OCT.
29	III	A.D. III KAL. OCT.
30	PRIDIE	PRID. KAL. OCT.
October	OCTOBER	OCT.
1	KALENDS	KAL. OCT.
2	VI	A.D. VI NON. OCT.
3	V	A.D. V NON. OCT.
4	IV	A.D. IV NON. OCT.
5	III	A.D. III NON. OCT.
6	PRIDIE	PRID. OCT.
7	NONES	NON. OCT.
8	VIII	A.D. VIII ID. OCT.
9	VII	A.D. VII ID. OCT.
10	VI	A.D. VI ID. OCT.
11	V	A.D. V ID. OCT.
12	IV	A.D. IV ID. OCT.
13	III	A.D. III ID. OCT.
14	PRIDIE	PRID. ID. OCT.
15	IDES	ID. OCT.
16	XVII	A.D. XVII KAL. NOV.
17	XVI	A.D. XVI KAL. NOV.
18	XV	A.D. XV KAL. NOV.
19	XIV	A.D. XIV KAL. NOV.
20	XIII	A.D. XIII KAL. NOV.
21	XII	A.D. XII KAL. NOV.
22	XI	A.D. XI KAL. NOV.
23	X	A.D. X KAL. NOV.
24	IX	A.D. IX KAL. NOV.
25	VIII	A.D. VIII KAL. NOV.
26	VII	A.D. VII KAL. NOV.
27	VI	A.D. VI KAL. NOV.
28	V	A.D. V KAL. NOV.
29	IV	A.D. IV KAL. NOV.
30	III	A.D. III KAL. NOV.
31	PRIDIE	PRID. KAL. NOV.
November	NOVEMBER	NOV.
1	KALENDS	KAL. NOV.
2	IV	A.D. IV NON. NOV.
3	III	A.D. III NON. NOV.
4	PRIDIE	PRID. NON. NOV.
5	NONES	NON. NOV.
6	VIII	A.D. VIII ID. NOV.
7	VII	A.D. VII ID. NOV.
8	VI	A.D. VI ID. NOV.
9	V	A.D. V ID. NOV.
10	IV	A.D. IV ID. NOV.
11	III	A.D. III ID. NOV.
12	PRIDIE	PRID. ID. NOV.
13	IDES	ID. NOV.
14	XVIII	A.D. XVIII KAL. DEC.
15	XVII	A.D. XVII KAL. DEC.
16	XVI	A.D. XVI KAL. DEC.
17	XV	A.D. XV KAL. DEC.
18	XIV	A.D. XIV KAL. DEC.
19	XIII	A.D. XIII KAL. DEC.
20	XII	A.D. XII KAL. DEC.
21	XI	A.D. XI KAL. DEC.
22	X	A.D. X KAL. DEC.
23	IX	A.D. IX KAL. DEC.
24	VIII	A.D. VIII KAL. DEC.
25	VII	A.D. VII KAL. DEC.
26	VI	A.D. VI KAL. DEC.
27	V	A.D. V KAL. DEC.
28	IV	A.D. IV KAL. DEC.
29	III	A.D. III KAL. DEC.
30	PRIDIE	PRID. KAL. DEC.
December	DECEMBER	DEC.
1	KALENDS	KAL. DEC.
2	IV	A.D. IV NON. DEC.
3	III	A.D. III NON. DEC.
4	PRIDIE	PRID. NON. DEC.
5	NONES	NON. DEC.
6	VIII	A.D. VIII ID. DEC.
7	VII	A.D. VII ID. DEC.
8	VI	A.D. VI ID. DEC.
9	V	A.D. V ID. DEC.
10	IV	A.D. IV ID. DEC.
11	III	A.D. III ID. DEC.
12	PRIDIE	PRID. ID. DEC.
13	IDES	ID. DEC.
14	XIX	A.D. XIX KAL. IAN.
15	XVIII	A.D. XVIII KAL. IAN.
16	XVII	A.D. XVII KAL. IAN.
17	XVI	A.D. XVI KAL. IAN.
18	XV	A.D. XV KAL. IAN.
19	XIV	A.D. XIV KAL. IAN.
20	XIII	A.D. XIII KAL. IAN.
21	XII	A.D. XII KAL. IAN.
22	XI	A.D. XI KAL. IAN.
23	X	A.D. X KAL. IAN.
24	IX	A.D. IX KAL. IAN.
25	VIII	A.D. VIII KAL. IAN.
26	VII	A.D. VII KAL. IAN.
27	VI	A.D. VI KAL. IAN.
28	V	A.D. V KAL. IAN.
29	IV	A.D. IV KAL. IAN.
30	III	A.D. III KAL. IAN.
31	PRIDIE	PRID. KAL. IAN.