How to convert CHAR values into DATETIME values in SQL Server (69133)

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To begin with, the character date and time must be converted into the SQL Server datetime format. Then the SMALLINT value can be subtracted to obtain START_TIME. Both tasks can be performed in a single SQL expression; however, it is easier to understand the process if both tasks are considered separately.

For example, if the time value is stored in a column named "term_time" with a format of HHMMSS, and the date value is stored in a column named "term_date" with a format of YYMMDD, these values can be converted to the SQL Server datetime with the following call:

   convert(datetime,term_date+" "+
          substring(term_time,1,2)+":"+
          substring(term_time,3,2)+":"+
          substring(term_time,5,2)   )
				

Given a date in SQL Server datetime format, it is easy to add or subtract a given amount of seconds, minutes, days, and so forth. For example, assuming the SMALLINT value ("@delta") is in seconds, the following function will return the start date/time:

   dateadd(ss,-(@delta),???)
				

The entire expression can be put into the following select statement so it is executed for each row in the input table. "@delta" is assumed to be an input parameter. If "@delta" is in units other than seconds, the first parameter of dateadd must be changed to reflect the correct units (minutes, days, and so forth).

   select  dateadd( ss, -(@delta), convert(datetime,term_date+" "+
               substring(term_time,1,2)+":"+
               substring(term_time,3,2)+":"+
               substring(term_time,5,2)   ))
     from t1