I use the full date and time to give each file a unique number that will never duplicate. Also helps with sorting and trouble shooting having the time in the file name rather than having to go to the fits header. I do end up with long file names though. 
The pattern for this is: _%yy%mm%dd%tm
An example created this way: ngc1808group_120sec_Clear@-15.0C_20160210200758
Clear Skies
Mick