At the bottom of the Control & Performance editor of the OT click Yes adjacent to "Are the observations time-constrained"? 

Once this is selected you can choose between two types of time-constrained observations:

• Single visit: Multiple time windows can be entered, during one of which the Science Goal should be executed.
  It is possible to import these windows from an external ASCII file. Each visit should go on a separate
  row and each row should contain two comma-separated values of format YYYY-MM-DDThh:mm:ss
  e.g.
  2015-03-19T01:30:00, 2015-03-19T01:30:00
  2015-03-21T01:30:00, 2015-03-21T01:30:00
  2015-03-23T01:30:00, 2015-03-23T01:30:00
  2015-03-25T01:30:00, 2015-03-25T01:30:00

• Multiple visits: Multiple dates (either absolute or relative to one of the epochs) at each of which
  the Science Goal should be executed.

Please refer to the relevant section (A.10 Time Constrained Observations) of the Proposer's Guide for more information on time-critical and monitoring observation.

Related Knowledgebase articles: 

• Can I set up monitoring observations with the OT?

• Can I specify the date on which my observations should be taken or set up a monitoring observation?