Date / Time System fields - inconsistent

Question

This is a weird one...please bear with me
Before I begin...note that this is not a problem that is difficult to fix, I'm just interested in better understanding why the system fields are behaving the way they are.
We have 20 or so programs that use a function module to create a file header that includes a date/time stamp.  The programs also place a trailer with a date/time stamp that should match the one in the header.  However, the programmer used different system fields (after a GET TIME statement) to create the date/time stamp in the trailer.  On occasion, the date/time stamp comes out different. 
Here's the code (sry about all caps):
  GET TIME.
  CONCATENATE SY-DATUM SY-UZEIT INTO BATCHID.
  CALL FUNCTION 'Z_YADAYADA'
       EXPORTING
            FILE_ID              = P_FILEID
            REFRESH_TYPE         = P_REFTYP
            START_PERIOD         = START_PER
            END_PERIOD           = END_PER
       IMPORTING
            STRING255            = OUTPUTSTRING
       EXCEPTIONS
            INVALID_REFRESH_TYPE = 1
            INVALID_BATCH_ID     = 2.
  ...sy-subrc check...
  TRANSFER OUTPUTSTRING TO P_FILE1.  <--header output
  CONCATENATE L_TRL BATCHID COUNT T_TOTAL SPACE
              INTO OUTPUTSTRING SEPARATED BY L_DEL.
  CONDENSE OUTPUTSTRING NO-GAPS.
  TRANSFER OUTPUTSTRING TO P_FILE1.  <--trailer output
inside function module *****
  CONCATENATE: SY-DATLO SY-TIMLO INTO DATESTAMP,
               SY-SYSID SY-MANDT INTO MACHINE_NAME.
  CONDENSE: DATESTAMP, MACHINE_NAME NO-GAPS.
  CONCATENATE C_HDR DATESTAMP FILE_ID MACHINE_NAME
              SY-CPROG ' ' REFRESH_TYPE START_PERIOD
              END_PERIOD DATESTAMP
              INTO STRING255 SEPARATED BY '~'.
  CONDENSE: STRING255 NO-GAPS.
You can see that the function uses SY-DATLO and SY-TIMLO for creation of the header, while SY-DATUM and SY-UZEIT are used for the trailer.  What's wierd is that sometimes the trailer has a date/time stamp that is 1 second <b>greater</b> than the header, despite the fact that it is stored in the BATCHID variable <b>before</b> the header is created.
Any help, references, explanations greatly appreciated (and rewarded with points)
BMV

Answer 1

Hi Brian
Here is the related part from the weblog of ABAP expert Horst Keller. Hope it may help. For the full document, you can visit <a href="https://www.sdn.sap.comhttp://www.sdn.sap.comhttp://www.sdn.sap.com/irj/servlet/prt/portal/prtroot/com.sap.sdn.wcm.compound.docs/library/uuid/f1391fc3-0301-0010-d384-fccd036f1c67">here</a>.
The values of all system fields in this table are implicitly set when the program is started, every time a screen layout of a screen is sent, and when the internal mode is set. The GET TIME command explicitly updates the system fields, except for sy-dayst, sy-fdayw and sy-tzone.
With the exception of sy-datlo and sy-timlo, all system fields refer to the local date and time of the current SAP system. The ABAP runtime environment clock is synchronized with the database server clock at regular intervals in order to calculate the local time of the SAP system. During the synchronization process, the ABAP runtime environment clock is set to the database server clock. Because this happens on all application servers in an SAP system, the ABAP runtime environment clock is synchronous with the clocks on all other application servers and with the database system clock, and thus shows the local time of the entire SAP system. The time zone on which the local time of an SAP system is based is the only entry in the database table TTZCU.
The content of sy-zonlo is taken from the user master record of the current user. The values of sy-datlo and sy-timlo are calculated from sy-datum and sy-uzeit and from the time zone of the SAP system for the time zone in sy-zonlo. If the user master record does not contain a time zone, or if it contains an invalid or an inactive time zone, sy-datlo and sy-timlo are set to the values of sy-datum and sy-uzeit. All valid time zones are defined in table TTZZ.
Is't the naming of sy-datum and sy-uzeit really geeky?
Time Stamps
The above system fields for date and time are not sufficient for many requirements of determining unique points in time: They represent local times and the values are measured in seconds. For more exact date and time determination, you use time stamps.
A time stamp represents date and time in the form YYYYMMDDHHMMSS. YYYY is the year, MM the month, DD the day, HH the hour, MM the minutes and SS the seconds. There is a short form and a long form. In the long form, the format specified above additionally contains 7 decimal places for fractions of seconds, which allows for an accuracy of up to 100 ns. The maximum time resolution depends on the operating system of the application server and may be less.
A valid time stamp must contain values whose date and time specifications before the decimal separator correspond to valid values for the data types d and t. Time stamps in this form are always considered as UTC (Universal Time Coordinated, basis for calculating worldwide time specifications; the UTC reference time is based on Greenwich Mean Time, GMT, but is not a time zone; it has no daylight saving time or summer time) time stamps when processed with the corresponding statements. You use the statement GET TIME STAMP to create a time stamp that represents the current UTC reference time.
Regards
*--Serdar <a href="https://www.sdn.sap.com:443http://www.sdn.sap.comhttp://www.sdn.sap.com/irj/servlet/prt/portal/prtroot/com.sap.sdn.businesscard.sdnbusinesscard?u=qbk%2bsag%2bjiw%3d">[ BC ]</a>