LB: The Hebrew calendar is based on months alternating between 29 and 30 days. 10 months are fixed; 2 are flexible. In a non-leap year Nissan (30), Iyar (29), Sivan (30), Tamuz, (29), Menachem Av (30), Elul (29), Tishrei (30), Marcheshvan (29 or 30), Kislaiv (29 or 30), Teves (29), Shvat (30), Adar (29). In a leap year Adar Rishon (30) and Adar Sheni (29).
This year (5777) both Marcheshvan and Kislaiv had only 29 days each therefore Asarah B’teves was on the 98th day of the year (30+29+29+10=98).
If Marcheshvan has 29 days and Kislaiv 30 says then it is the 99th day of the year (30+29+30+10=99).
If Marcheshvan has 30 days and Kislaiv 30 days then it is the 100th day of the year (30+30+30+10=100).