Date

TIMESTAMP=`date +%s%N`

Pretty unique timestamp (seconds since 1-1-1970 and current nanosecond)

TIMESTAMP=`date +%Y%m%d_%H%M`

Common timestamp (Year, month, day, hour, minute)

YESTERDAY=`date -d "yesterday 13:00" +%Y%m%d`

DST safe way to get yesterday, just using yesterday between 23:00 and 01:00 can have unwanted result.