Hi,
this has to do with the scaling and formating you are using.
If you are using an automatic Scaling mode and you X-values have the display format "Time", DIAdem switches to the scaling type "Date/Time".
By default, DIAdem is using an automatic formating with two lines. The upper is for the Ticks and the lower for a larger step. It looks as if your time-data is only holdig the seconds of the day, but not the date itself. So DIAdem assumes a year 0 and the 1 is for the first day of the year. If your data spans over several days, you will see the number of the day below the 12:00 for each day. Depending on the X-values, the automatic scaling will show seconds, minutes, hours or dates and years.
You just need to go to the X-Axis - numbers tab and change the format. "#hh:nn" would do the job with leading zeros and "#HH:nn" without leading zeros.