/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.commons.text.lookup;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

/**
 * Formats the current date or the date in the LogEvent. The "key" is used as the format String.
 */
final class DateStringLookup extends AbstractStringLookup {

    /**
     * Defines the singleton for this class.
     */
    static final DateStringLookup INSTANCE = new DateStringLookup();

    /**
     * No need to build instances for now.
     */
    private DateStringLookup() {
        // empty
    }

    /**
     * Formats the given {@code date} long with the given {@code format}.
     *
     * @param date
     *            the date to format
     * @param format
     *            the format string for {@link SimpleDateFormat}.
     * @return the formatted date
     */
    private String formatDate(final long date, final String format) {
        DateFormat dateFormat = null;
        if (format != null) {
            try {
                dateFormat = new SimpleDateFormat(format);
            } catch (final Exception ex) {
                throw IllegalArgumentExceptions.format(ex, "Invalid date format: [%s], using default", format);
            }
        }
        if (dateFormat == null) {
            dateFormat = DateFormat.getInstance();
        }
        return dateFormat.format(new Date(date));
    }

    /**
     * Looks up the value of the environment variable.
     *
     * @param key
     *            the format to use. If null, the default {@link DateFormat} will be used.
     * @return The value of the environment variable.
     */
    @Override
    public String lookup(final String key) {
        return formatDate(System.currentTimeMillis(), key);
    }
}

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.commons.text.lookup;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;

import org.apache.commons.lang3.time.FastDateFormat;

/**
 * Formats the current date or the date in the LogEvent. The "key" is used as the format String.
 */
final class DateStringLookup extends AbstractStringLookup {

    /**
     * Defines the singleton for this class.
     */
    static final DateStringLookup INSTANCE = new DateStringLookup();

    /**
     * No need to build instances for now.
     */
    private DateStringLookup() {
        // empty
    }

    /**
     * Formats the given {@code date} long with the given {@code format}.
     *
     * @param date
     *            the date to format
     * @param format
     *            the format string for {@link SimpleDateFormat}.
     * @return the formatted date
     */
    private String formatDate(final long date, final String format) {
        FastDateFormat dateFormat = null;
        if (format != null) {
            try {
                dateFormat = FastDateFormat.getInstance(format);
            } catch (final Exception ex) {
                throw IllegalArgumentExceptions.format(ex, "Invalid date format: [%s], using default", format);
            }
        }
        if (dateFormat == null) {
            dateFormat = FastDateFormat.getInstance();
        }
        return dateFormat.format(new Date(date));
    }

    /**
     * Looks up the value of the environment variable.
     *
     * @param key
     *            the format to use. If null, the default {@link DateFormat} will be used.
     * @return The value of the environment variable.
     */
    @Override
    public String lookup(final String key) {
        return formatDate(System.currentTimeMillis(), key);
    }
}

Add an interpolator string lookup. Reuse Apache Commons Lang's FastDateFormat.