I played around with this for practise. The 'correct' answer is what Christian Strempfer posted - this is what pub.date:dateTimeFormat is meant to do: transform beween datetime string formats. I'm not sure about your date pattern though - try currentPattern=yyyy-MM-dd HH:mm:ss.SS and newPattern=yyyy-MM-dd
A 'good hacky approach is using pub.string:subString (positions 0 and 10) to simply hack the end off the input string. You can also try regexs -- pub.string:replace, useRegex=true, searchString=^(.{10}).*, replaceString=$1. (searchString=^(.{10}) should also work, but it doesn't)