while selecting least date, have to exclude bland fields

I could select least date among 5 date fields...

Problem is 1 or 2 fields are not having data...so the result is coming as blank... I have to find the least among available dates...

My present code is like this:

FORM LEAST_DAT_CALC .

DATE7 = I_PA9030-ZDT_TMPSERV.

IF DATE7 > I_PA9030-ZDT_TRG.

DATE7 = I_PA9030-ZDT_TRG.

ENDIF.

IF DATE7 > I_PA9030-ZDT_WRKCHG.

DATE7 = I_PA9030-ZDT_WRKCHG.

ENDIF.

IF DATE7 > I_PA9030-ZDT_TSKFRC.

DATE7 = I_PA9030-ZDT_TSKFRC.

ENDIF.

IF DATE7 > I_PA9030-ZDT_PERSERV.

DATE7 = I_PA9030-ZDT_PERSERV.

ENDIF.

ENDFORM. " LEAST_DAT_CALC

Pls help me...