Why? Get your clearance when the flight plan is in the ATC system. Typically, that is 30 minutes prior to one hour prior to the proposed time.

If ATC is not present on the field, you'll get a "hold for release" at the end of your clearance.

FAA Order 7110.65

4-3-6

PHRASEOLOGY-
(Aircraft identification) RELEASED FOR DEPARTURE
AT (time in hours and/or minutes),
and if required,
IF NOT OFF BY (time), ADVISE (facility) NOT LATER
THAN (time) OF INTENTIONS.
TIME (time in hours, minutes, and nearest quarter minute).
d. When expect departure clearance times
(EDCT) are assigned through traffic management
programs, the departure terminal must, to the extent
possible, plan ground movement of aircraft destined
to the affected airport(s) so that flights are sequenced
to depart no earlier than 5 minutes before, and no later
than 5 minutes after the EDCT. Do not release aircraft
on their assigned EDCT if a ground stop (GS)
applicable to that aircraft is in effect, unless approval
has been received from the originator of the GS.
1. If an aircraft has begun to taxi or requests taxi
in a manner consistent with meeting the EDCT, the
aircraft shall be released. Additional coordination is
not required.
2. If an aircraft requests taxi or clearance for
departure inconsistent with meeting the EDCT
window, ask the pilot to verify the EDCT.
(a) If the pilot's EDCT is the same as the FAA
EDCT, the aircraft is released consistent with the
EDCT.
(b) If the pilot's EDCT is not the same as the
FAA EDCT, refer to Trust and Verify Note below.
3. If an aircraft requests taxi too late to meet the
EDCT, contact the ATCSCC through the appropriate
TMU

3-9-2. DEPARTURE DELAY INFORMATION
USA/USAF/USN NOT APPLICABLE
When gate‐hold procedures are in effect, issue the
following departure delay information as appropriate:
REFERENCEFAAO
JO 7210.3, Para 10-4-3, Gate Hold Procedures.
a. Advise departing aircraft the time at which the
pilot can expect to receive engine startup advisory.
PHRASEOLOGYGATE
HOLD PROCEDURES ARE IN EFFECT. ALL
AIRCRAFT CONTACT (position) ON (frequency) FOR
ENGINE START TIME. EXPECT ENGINE START/TAXI
(time).
b. Advise departing aircraft when to start engines
and/or to advise when ready to taxi.
PHRASEOLOGYSTART
ENGINES, ADVISE WHEN READY TO TAXI,
or
ADVISE WHEN READY TO TAXI.
c. If the pilot requests to hold in a delay absorbing
area, the request shall be approved if space and traffic
conditions permit.
d. Advise all aircraft on GC/FD frequency upon
termination of gate hold procedures.
PHRASEOLOGYGATE
HOLD PROCEDURES NO LONGER IN EFFECT.