Convert bigtime and bigdatetime Data

Implicit and explicit conversions are allowed where a decreased precision results in the loss of data.

Implicit conversion between types without matching primary fields may cause either data truncation, the insertion of a default value, or an error message to be raised. For example, when a bigdatetime value is converted to a date value, the time portion is truncated leaving only the date portion. If a bigtime value is converted to a bigdatetime value, a default date portion of Jan 1, 0001 is added to the new bigdatetime value. If a date value is converted to a bigdatetime value, a default time portion of 00:00:00.000000 is added to the bigdatetime value.