I have used the built in clock many times over the years. BEWARE!... The clock is not a Swiss time piece; I would suggest that you plan on correcting the time 4 times a year. Since the internal clock does not have a daylight savings time function, updating time at least 2 times a year (if you use daylight savings time) is necessary. Plan on checking the time half-way between the daylight savings days. I have found each PLC is slightly different in its accuracy; however I found 2-3 minutes shift over a 6 months period. If you have an HMI, time setting can be done through it using PLC logic.