Field 045 contains codes which specify a time period associated with the material or subject matter in a bibliographic record. This field is described at
Although this field is now available for many types of materials, it has been used most often in the cataloging of musical scores and sound recordings, to indicate the dates when their contents were composed.
Subfield |a contains a four-character code, with two characters each for the beginning and end of the time period expressed. This 045A search finds materials relating to the 1890s:
Subfield |b specifies a date, with at least one year of precision. It may include a month, a day, and an hour. A single 045 field may contain multiple subfields |b, the significance of each being specified by the second indicator (1 for a list of times, 2 for a range of time). Keyword searching is, unfortunately, not able to reflect these refinements. The following example of a 045B search finds instances of the year 1945 having been coded (either as a specific date, a beginning date, or an ending date). The question-mark wild-card causes the search to include all more specific periods in the year.
A single 045 field may contain both types of coding, in both subfields |a and |b.