Thanks for the information Daniel.
While looking through the FTP / MFS code to see where I could place this I noticed that MFS already tries to write the date based on _time_get. Then it dawned on me that if I synchronised the systems tick with the real time it should all work. So I have now added the following to my start-up code:
//Synchronise systems clock to real time clock
rtc_datetime_t datetime;
RTC_DRV_GetDatetime( FSL_RTCTIMER1, &datetime );
DATE_STRUCT date;
TIME_STRUCT time;
date.YEAR = datetime.year;
date.MONTH = datetime.month;
date.DAY = datetime.day;
date.HOUR = datetime.hour;
date.SECOND = datetime.second;
date.MILLISEC = 0;
_time_from_date(&date, &time);
_time_set(&time);
I'm not sure if that is the most efficient way but it works and now the date/time of any files I create use the current real time.
Adrian.