Select the checkbox next to a specimen you have collected, or you can let the system do this for you after you have entered the date.
Select the Calendar icon to record date and time of collection.

Select the checkbox next to the specimen you have collected and system will populate the collection date with current day's date and time. You may, however, change the date and time later
Alternatively, select the calendar icon beside the collection date field after which system displays a calendar where you may select the date and time of collection

Record Collection date by selecting the desired date from calendar. To select time of collection, drag the sliders to adjust hours and minutes in time. If you have collected the specimen right now, select Now in the calendar and the Done.
Following is a screenshot indicating how the checklist view looks once Collection date is recorded.

Once date and time of collection are entered, system selects the checkbox to indicate you have captured the collection date of that specimen. However, status of the specimen will still be Pending until You select Save located on top/bottom right corner to mark the specimen as collected.
Status of the specimen is updated from Pending to Collected after you save Specimen checklist of that patient