To filter the activities on the Day view, use filter textbox at the bottom.

You can only filter on one timeline at the time. For example to just display those activities which have 'google' in the title, click on Applications timeline to select it, then in filter write 'google'.

Search in tags Notes field
When tag timeline is selected, filter will also work on the Notes field. 

Filter on Day view only works on specific date, to search activities over more than one day, use Advanced search. 

Keywords

There are some keywords you can use to make filter more efficient:

• group - search for words in group
  

  usage: group:firefox - all Firefox activities

• duration - how long an activity can be
  

  usage: duration>1s - duration is longer than 1 second
  duration<1m - duration is shorter than 1 minute
  duration>1m10s - duration is longer than 1 minute and 10 seconds
  duration<1h - duration is shorter than 1 hour

• start - when activity started
  

  usage: start>10:00PM - activity started after 10:00 PM
  start>22:00 - activity started after 10:00 PM or 22:00

• end - when activity ended
  

  usage: end>10:00PM - activity ended after 10:00 PM
  end>22:00 - activity ended after 10:00 PM or 22:00

• or
  

  usage: end>10:00PM or duration<10s

• and - and is implicit operator so you don't need to use it
• - - not operator
  

  usage: 'manictime' will look for all lines with text manictime in title, '-manictime' will show all lines except those which contain manictime text in title

• label - tags can have labels, currently the only label is billable
  

  usage: label=billable

Wildcard characters

There are two wildcard characters available: ? and *:
? will substitute any single non white-space character
* will substitute any number of non white-space characters

Let say I have these lines:
Cars
Cats

If I write Ca?s it will match both lines.
If I write C*s it will also match both lines.

Examples

A few examples for Applications timeline:

google - find all activities with keyword 'google' in title

-google - find all activities without keyword 'google' in title

group:firefox - find all activities with keyword 'firefox' in group

group:note - find all activities with keyword 'note' in group. For example, if I am using Notepad and Evernote all instances where either of these two applications was used will match.

group:firefox google - find all activities with keyword 'firefox' in group and word 'google' in title

group:"google chrome" (microsoft or yahoo) - find all activities with keyword 'google chrome' in group and word 'microsoft' or 'yahoo' in title

group:"google chrome" (microsoft or yahoo) start<13:00 - find all activities with keyword 'google chrome' in group and word 'microsoft' or 'yahoo' in title, starting before 13:00

manictime duration>1m end<6:00PM - find all activities with keyword 'manictime' in title, ending before 6PM and longer than 1 minute

note:"some note" - find all tags with keyword 'some note' in note

Regular expressions

Instead of only text and wildcard characters, you can also use regular expressions. To use regular expressions, encapsulate them in
#"regular expression"
For example, to search for anything:
#".*"