Old Style calendar and Solar calendar

Old Style calendar noun - The solar calendar introduced in Rome in 46 b.c. by Julius Caesar and slightly modified by Augustus, establishing the 12-month year of 365 days with each 4th year having 366 days and the months having 31 or 30 days except for February.

Solar calendar and Old Style calendar are semantically related. In some cases you can use "Solar calendar" instead a noun phrase "Old Style calendar".

Solar calendar noun - A calendar based on solar cycles.

Old Style calendar and solar calendar are semantically related. Sometimes you can replace phrase "Old Style calendar" with "Solar calendar", this phrases are similar.