question: @ Nullable public Date convert ( @ Nullable Date srcDate , TimeZone srcTimeZone , TimeZone dstTimeZone ) { if ( srcDate == null ) return null ; Preconditions . checkNotNullArgument ( srcTimeZone , "srcTimeZone is null" ) ; Preconditions . checkNotNullArgument ( dstTimeZone , "dstTimeZone is null" ) ; int srcOffset = srcTimeZone . getOffset ( srcDate . getTime ( ) ) ; int dstOffset = dstTimeZone . getOffset ( srcDate . getTime ( ) ) ; return new Date ( srcDate . getTime ( ) - srcOffset + dstOffset ) ; }
target:  Converts date between time zones.